WASHINGTON (AP) Retaliating 13 days after the deadly embassy bombings in East Africa, U.S. forces Thursday launched cruise missile strikes against alleged terrorist camps in Afghanistan and a chemical plant in Sudan.

The thunderclap of strikes signaled what president Bill Clinton said would be a lengthy war on terrorists who bombed two U.S. embassies in Kenya and Tanzania Aug.7 and were allegedly planning still more attacks.

Calling Thursday's strike a pre-emptive measure, U.S. officials nevertheless warned that the military action almost certainly would provoke a terrorist response.

Some U.S. airports tightened security immediately, and the CIA told senior Clinton administration officials that the prospect of retaliation against Americans overseas was "very, very high." The State Department urged U.S. citizens traveling or residing abroad to remain alert and to exercise much greater caution than they usually would.

The FBI issued a nationwide alert to U.S. law enforcement officials to watch for signs of terrorist action.

Interrupting his vacation on Martha's Vineyard in Massachusetts, Clinton jolted the press corps and the nation from the scandal over his relationship with a former White House intern with the surprise announcement of the strikes, which took place at 1:30 p.m. EDT.

U.S. ships in the Red and Arabian seas fired 75 to 80 Tomahawk cruise missiles at a suspected chemical weapons plant in Khartoum, Sudan, and a terrorist training complex in eastern Afghanistan near the Pakistani border.

Smoke, flames and smoldering ruins signaled the destruction of the suspected chemical plant in Sudan. Pentagon officials said it might take days to assess the damage caused by strikes in Afghanistan at the sprawling terrorist complex, which encompassed six separate targets in remote, arid countryside.

A spokesman for the Taliban religious army, which controls most of Afghanistan, said 21 people died and another 30 people were injured when cruise missiles exploded on the Zhawar Kili Al- Badr base near Khost, about 150 km southeast of the capital, Kabul.

Wakil Ahmed Akhumzada, reached by telephone in the southern city of Kandahar, said another 30 people were injured.

"It is such a callous act," Akhumzada said. "They have no proof of terrorism committed by (Osama) bin Laden or the Taliban. If they were going to take such action, they should have contacted us first."

Akhumzada also said missiles hit targets near Jalalabad, about 100 km east of Kabul. The deputy governor of the region had said three people were killed in the Jalalabad attack. U.S. defense officials had said only the Al-Badr base was targeted, and six sites were hit on that base.

The Pakistani mews agency NNI, quoting the Harkat-ul- Mujahedeen militant group, said five Pakistanis died in then strike against Al-Badr. It was not known if Akhumzada was including the five Pakistanis and the reports could not be immediately confirmed.

Sudanese officials said at least seven workers at the alleged chemical plant were injured in the missile strikes there.

Sudanese President Omar el-Bashir said his country "reserves the right to respond to the American attack using all necessary measures," Egypt's Middle East mews agency reported Friday.

The strikes apparently left unharmed the man in control of these sites, Osama bin Laden, the Saudi multimillionaire linked over the past week to the bombing of the embassies in Kenya and Tanzania that killed a dozen Americans and more than 200 Africans.

Even before the dust settled, and as Clinton hurried back to Washington to address the nation from the Oval Office, the outlines of a harder U.S. line on terrorism were clear.

"We will not yield to this threat. We will meet it no matter how long it may take," Clinton said, "This will be a long, ongoing struggle."

Senior Clinton administration officials underscored the point.

"This is not simply a response to some specific act," said Army Gen. Henry Shelton, chairman of the Joint Chiefs of Staff, "but a concerted effort to defend U.S. interests."

At United Nations headquarters in New York, Ambassador Bill Richardson informed the Security Council that the United States acted in self-defense, in accord with the U.N. charter.

Secretary of State Madeleine Albright, who led the push within the administration to strike, said the action demonstrates "there will be no safe haven for terrorist."

Patrick Cronin, a terrorism expert at the U.S. Institute of Peace, said the bin Laden group "was one of the key terrorist organizations known to the Western intelligence community." While the attacks may disrupt bin Laden's plans in the short term, Cronin said, "we have to be prepared against retaliation."

A few Republicans suggested Clinton -- due to return to Martha's Vineyard Friday -- timed the strike to deflect attention from his admission Monday that for seven months he had misled the nation by denying a sexual relationship with Monica Lewinsky.

"I'm not saying that force is not appropriate," said Republican Sen. Dan Coats. "I'm questioning the timing."

The White House angrily denied that charge.

Most senior Republicans, including House Speaker Newt Gingrich and Sen. Jesse Helms, praised the strikes, calling them necessary and even overdue.

In Sudan, protesters stormed the empty U.S. Embassy compound in Khartounm, throwing stones and shouting "Down, Down USA!" The embassy was abandoned in 1996, after the United States decided it could not protect diplomats there.

Increasingly firm intelligence collected by the CIA since the embassy bombings formed the basis of Clinton's decision, senior administration officials said.

The El Shifa Pharmaceutical Plant in Khartoum, said by Sudanese officials to make antibiotics, was believed by the CIA to produce the key ingredient in the deadly nerve gas VX.

The sites in eastern Afghanistan made up what a senior intelligence official called the largest and most extensive Sunni Muslim "terrorist university" in the world.

It included firing ranges, indoctrination centers, tanks and armored personnel carriers and stores of ammunition and explosives.

As evidence grew that bin Laden was behind the embassy attacks, multiple intelligence sources indicated that bin Laden's group was on the verge of launching attacks on other U.S. embassies.

Late last week, information collected by the CIA indicated that senior Islamic militant leaders planned to gather at the Afghan training center Thursday, a key reason why Clinton chose that day to strike.

Clinton approved the strike in principle a week ago and gave the final go-ahead early Thursday.

U.N. staffers shot

ROME (Reuters) An Italian military officer and a French national working for the United Nations in Afghanistan were shot while riding in a U.N. vehicle in Kabul Friday, Italian and French officials said.

In Rome, the office of the Defense Ministry joint chiefs of staff said the Italian officer, 42-year-old Carmine Calo, was hit by a bullet in the chest. He was undergoing surgery.

In Paris, the Foreign Ministry reported a French national working for the same U.N. mission was also wounded in the incident.

U.S. Constitution no barrier to raid

WASHINGTON (AP) President Bill Clinton's military strikes against a terrorist camp in Afghanistan and a chemical plant in Sudan were authorized under the U.S. Constitution and a 1996 antiterrorist law, the Justice Department stated Thursday.

"The President acted pursuant to his constitutional authority, including that as commander in chief and his authority, to protect national security," said Justice Department spokesman Bert Brandenburg.

Brandenburg also cited as authority for the strikes the Anti-Terrorism and Effective Death Penalty Act passed in 1996 after the Oklahoma City federal building was bombed in 1995.

That act states, "The President should use all necessary means, including covert action and military force, to disrupt, dismantle and destroy international infrastructure used by international terrorists, including overseas terrorist training facilities and safe havens."