OTTAWA -- The federal government went on a security alert and shut down all airports except for diverted aircraft in response to the terrorist attacks in the United States yesterday, but a shaken Prime Minister Jean Chrétien said these are precautions and there is no specific threat to Canada.

RCMP Commissioner Giuliano Zaccardelli said the Mounties are co-operating with U.S. authorities. He said his force is investigating whether Canada was a staging area for the attacks, but added that there is no evidence yet of any Canadian connection.

Mr. Chrétien, who said he was horrified by the attacks in New York and Washington, offered whatever Canadian emergency assistance might be needed.

The first indication in the Prime Minister's Office that a serious terrorist attack was under way came in a telephone call from a Transport Canada official to Mr. Chrétien's chief of staff, Percy Downe, shortly after the first American Airlines 767 hit the south tower of the World Trade Center in New York.

After a brief debate among senior officials, the decision was made to interrupt a breakfast meeting the Prime Minister was having at his official residence with Saskatchewan Premier Lorne Calvert.

Stewards were just clearing away the creamy, gold-embossed plates, and the two leaders -- along with policy adviser Eddie Goldenberg and federal-provincial-relations official Frank Patten -- had begun a discussion of the Saskatchewan drought. Then the Prime Minister's executive assistant, Bruce Hartley, came into the room and outlined the limited information available at the time.

The decision was made, probably on the advice of the RCMP, that Mr. Chrétien should remain at 24 Sussex.

The Prime Minister and his officials crowded around a television set while they received a round of briefings from Chief of the Armed Forces General Ray Hénault, RCMP Commissioner Zaccardelli and the senior adviser from the Department of Foreign Affairs, Claude Laverdure.

He placed a call to U.S. President George W. Bush in Florida but could not reach him. Canadian Ambassador Michael Kergin, however, conveyed Ottawa's offer of assistance to senior U.S. officials.

The RCMP and Canadian Security Intelligence Service routinely work closely with the Federal Bureau of Investigation and the Central Intelligence Agency on terrorism cases. The Canadian agencies stepped up their activities yesterday to try to help the Americans identify the perpetrators, officials said.

Mr. Zaccardelli said security was heightened across the country, but that "there is no intelligence indicating that Canada was a target."

He played down the immediate threats to Canadians, but was quick to add that people around the world are always vulnerable to terrorist acts.

Some Canadian officials, noting that the Americans were quick to impose border security restrictions yesterday, feared for the long-term implications of the attacks. Canada wants to ease border-crossing procedures to facilitate trade. But many U.S. politicians have urged border clampdowns in response to terrorist threats.

The case of Ahmed Ressam, who is awaiting sentencing in the United States for trying to smuggle explosives into that country from British Columbia, is frequently cited by U.S. politicians arguing for a border clampdown. CSIS itself has warned that Canada makes an attractive staging area for terrorists intent on striking U.S. targets.

Although Mr. Zaccardelli said he had no knowledge of any Canadian connection to the latest bombings, he said the RCMP had learned a lot from the Ressam case and indicated his investigators would be searching for any possible links or similarities between the explosives smuggling plot and the New York and Washington bombings.

While officials waited for assessments of the threat, the Prime Minister cancelled a planned trip to Halifax, which included an announcement of a research grant and a speech to a party fundraiser.

Later in the day, Mr. Chrétien told a news conference in an eerily quiet Centre Block on Parliament Hill that there is no immediate word on whether any Canadians were killed or injured in New York or Washington. The Department of Foreign Affairs has a toll-free line for Canadians seeking whatever information might become available about friends and relatives visiting the cities. The number is 1-800-387-3124.

Parliament is in recess and thus the number of people working on Parliament Hill was greatly reduced. A rotating strike by public servants had already shut down some major government buildings in the capital yesterday morning. Military police stepped up security at defence headquarters and at bases across the country.

Mr. Chrétien cancelled his remaining official engagements, including a scheduled meeting today in Ottawa with the visiting Prime Minister of Slovakia and a trip to Sweden on the weekend for a summit meeting.

The attacks caught the Canadian government with many of its key ministers out of town on official business. Both Defence Minister Art Eggleton and Foreign Affairs Minister John Manley were travelling in Europe. Solicitor-General Lawrence MacAulay, the minister responsible for the RCMP and CSIS, was in Yarmouth, N.S. Transport Minister David Collenette drove back to Ottawa after ordering commercial air traffic grounded.

Officials said Mr. Chrétien kept in touch with his ministers by telephone, but there was no need for an emergency cabinet meeting.