Responding to heightened concern about foreign terrorism, law enforcement agencies yesterday tightened their grip at the nation's borders, increasing inspectors and redoubling security at airports.

And last night the State Department reemphasized its warning that Americans abroad may be the target of terrorist attacks in the next few weeks.

In Vermont, an Algerian man and a Canadian woman were arrested at a border crossing and charged with immigration violations after dogs found what may have been traces of explosives in their car. Authorities said that Bouabide Chamchi, 20, and Lucia Garofalo, 35, of Montreal, attempted to cross illegally from Canada into Beecher Falls, Vt., on Sunday night. Chamchi was carrying a false French passport. During the previous two weeks, Garofalo had brought another Algerian passenger across the border and tried unsuccessfully to drive across with a Pakistani-born man under circumstances authorities described as suspicious. Chamchi and Garofalo were charged in U.S. District Court in Burlington yesterday with conspiring to misuse a false French passport and various immigration offenses.

Canadian and U.S. authorities were trying to determine whether the pair was in any way connected to Algerian-born Ahmed Ressam, who was arrested in Port Angeles, Wash., on Dec. 14 after he arrived on a ferry from Canada with a carload of bomb-making materials.

Law enforcement officials said their chief focus is locating three possible accomplices of Ressam in the United States or Canada--a man he shared a motel room with in Vancouver, another man who may have been on the ferry with him and a third man whom officials have not described.

At the White House, increased concerns about possible terrorism tied to the end of the millennium prompted a high-level meeting Monday involving Secretary of State Madeleine K. Albright, CIA Director George J. Tenet, Gen. Henry H. Shelton, chairman of the Joint Chiefs of Staff, and others. President Clinton did not attend.

At his regular press briefing, White House spokesman Joe Lockhart said, "The U.S. is not invulnerable to domestic terrorism, but there's no specific information now about particular targets here."

Abroad, however, it is a different story. The State Department said last night that recent arrests of 13 people in Jordan produced information that hotels, tourist sites and tourist buses may have been targeted there and that even after the arrests "we cannot rule out the possibility that attacks still may be planned for this and other parts of the world."

Last night's warning tells Americans abroad to "review their security practices, to remain vigilant to their surroundings and to exercise caution" and urges them to "avoid large crowds and gatherings, keep a low profile, and vary routes and times of all required travel."

On Dec. 11, the State Department warned U.S. citizens to be especially careful through New Year's. That warning was apparently triggered in part by the arrests in Jordan, although no country was named then. Last night's stronger warning extended the caution period through mid-January.

Since Ressam's arrest last week, the U.S. Customs Service has transferred more than 300 employees to points of entry along the Canadian and Mexican borders, asked part-time employees who usually work during the summer tourist season to return to duty, and waived overtime limits so inspectors can work longer hours. There were reports of moderate border delays from the western seaboard to New England as inspectors asked more questions and stopped more travelers.

Customs Commissioner Raymond W. Kelly said travelers "may expect some delays. I believe they will be minimal . . . but we think it is important for us to intensify some of our activities."

The Immigration and Naturalization Service announced yesterday that it planned to institute similar measures in the coming days.

The Federal Aviation Administration also issued a statement outlining more stringent security precautions. The FAA said that passengers at airport security checkpoints may notice the presence of uniformed police and the additional use of trace explosives detector units. Passengers were advised not to gift wrap items.

[In Canada, the customs agency said it would step up security at airports, while the Royal Canadian Mounted Police confirmed an increase in security outside the U.S. Embassy in Ottawa, apparently in coordination with the U.S. actions, the Associated Press reported.]

Law enforcement officials cautioned that they have no evidence of a broad conspiracy linking Ressam and the Vermont arrest; nor have they established any firm evidence connecting the recent arrests with militant Islamic leader Osama bin Laden.

The Vermont arrest, however, was unusual. "We have a number of border crossing prosecutions that our office prosecutes every year," said Charles Tetzlaff, U.S. attorney for Vermont. "I think the thing that makes it a little unusual was the dogs alerting on the explosives residue--or whatever it is."

Two weeks before her arrest, Garofalo had entered the United States at the Pittsburg, N.H., port of entry on Dec. 6, according to an affidavit filed in Burlington federal court yesterday. Because the unmanned but controlled port of entry is located in a remote area, she was questioned over a video camera rather than face to face.

She drove a new Chrysler Neon and had two passengers who she told U.S. inspectors were Canadians. Inspectors did not ask each individual for identification, and all were allowed to enter the United States.

On Dec. 12, apparently returning, Garofalo crossed back into Canada at Stanstead, Quebec, above the Vermont border. Canadian Customs officials identified her two passengers as her son, and Mustafa Roubici, an Algerian citizen who had no identification. Garofalo said she had entered the United States with the same individuals on Dec. 6, suggesting that at that time she had misrepresented to U.S. officials that Roubici was Canadian.

On Dec. 15, Garofalo appeared again at the Pittsburg, N.H., video port of entry at 2:35 a.m. in the Neon. Her companion this time identified himself as Ahmed Saheen, who claimed to be a Canadian citizen born in Pakistan. A check of the trunk over the video revealed a large amount of baggage. At that point, they were refused entry because Saheen did not have a visa. Garofalo was told to go to a port of entry with inspectors present, but apparently did not.

Then, on Dec. 19, Garofalo and Chamchi drove to the port of entry at Beecher Falls, Vt., where the two were questioned. Asked why they were making the trip, Garofalo, according to the affidavit, said "she was looking for a restaurant in East Hereford, Quebec, the Canadian town immediately across the border." She said it was common for her to "take long pleasure rides with no specific purpose or destination."

Chamchi had no identification document required for an Algerian citizen in the United States, and a false French passport in the name of Boudjemaa Ben Ali was found in a jacket of his in the car. Chamchi said he had never seen it before. The FBI was notified and arrangements were made to have bomb-sniffing dogs inspect the vehicle. One dog was certified to detect plastic explosives, the other black powder explosives.

"The dogs were brought into Garofalo's car separately. Both dogs individually alerted in the right-rear area of the car's quarter panel by sitting. They also alerted to the portion of the trunk that adjoined the right rear quarter panel," the affidavit stated.

Canadian criminal records indicate that there was an outstanding warrant for Chamchi for an assault charge on Aug. 18 in Quebec. Chamchi and Garofalo, if convicted of the current charges against them, face sentences of up to five years imprisonment.

Staff writers Stephen Barr and Charles F. Babington contributed to this report.

CAPTION: In stepped-up security along southern and northern U.S. ports of entry, a Customs Service inspector empties the trunk of a car at the International Peace Bridge at Buffalo. Across the border in Canada, officials scrambled to explain how an Algerian terrorism suspect eluded them for nearly six years before his arrest Dec. 14 in the state of Washington. Details, Page A8.