OTTAWA, Dec. 18 - Fresh from a visit to Cuba, President Vladimir V. Putin of Russia visited Canada today and lined up its support for strengthening a cold war treaty that the Russians see as blocking an American national missile defense system.

"Canada and the Russian Federation agree that the 1972 Antiballistic Missile Treaty is a cornerstone of strategic stability," read the joint communiqué signed by Mr. Putin and by Prime Minister Jean Chrétien of Canada.

In a diplomatic offensive directed at American allies this year, the Russians have argued that the treaty rules out the kind of missile defense system advocated by President-elect George W. Bush.

Such a system, directed toward "rogue states" like North Korea or Iraq, could be operated without using Canada's Arctic airspace. But Canada's leader seemed unenthusiastic about the idea.

"Our preoccupation and everyone's preoccupation is to ensure that the stability that exists now is not undermined by this plan put forward by the Americans," Mr. Chrétien said at a joint news conference.

Before flying here on Sunday from Havana, Mr. Putin, a former K.G.B. officer, worked to soften his image with Canadians. In an advance interview with Canadian reporters, he recited National Hockey League statistics, mused about working as an environmentalist after serving as president, and disclosed that he is studying English. Today Mr. Putin said Canada could be a "mediator" between Russia and the United States over the missile system issue.

Last month, after Mr. Chrétien won a third term as prime minister, his aides noted that, with President Clinton's retirement in January, Mr. Chrétien will be the longest-serving leader among the world's seven major industrial nations and Russia.

With his Canada trip, Mr. Putin achieves his goal of meeting this year with every head of government of the group, which also includes Britain, France, Germany, Italy and Japan. Saving this trip for last, he seemed to be trying to tweak Washington.

Some experts on Russia see a bit of tit for tat for American diplomatic and commercial forays into Russia's "near abroad" - the southern belt of former Soviet republics that are now independent nations.

"Putin, by bringing a diplomatic offensive into America's backyard, has played a weak hand very deftly," Joseph Cirincione, nonproliferation director for the Carnegie Endowment for International Peace and a self-described "techno-skeptic" on the missile plan, said today from Washington. "The talks with Russians are going to be the way Powell described them on Saturday - tough negotiations," he said, referring to Gen. Colin L. Powell, tapped to be the Bush administration's secretary of state.

Mr. Chrétien said that the United States had not made any formal request for Canadian participation in the missile defense system: "We believe, at the moment, the question is more hypothetical because of the problems the Americans have had on technology.

Two of the most dynamic areas of cooperation between Russia and Canada were highlighted today. They are working to open up their space to send passenger jets over the polar region, which would generate million of dollars in new overflight fees. In the area of Arctic cooperation, Canada has spent $30 million on 45 development and aid projects for Russia's north since the end of the Soviet Union.