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Russia has said it will honour a ceasefire with Georgia starting Friday, but Georgia's president says that Russian tanks and other vehicles are advancing on his country's second-largest city.

Mikhail Saakashvili says that Russian forces now control a third of Georgia and has made allegations of "ethnic cleansing."

"I accuse the Russian government of ethnic cleansing and I can prove it . . . with the help of international organizations," Georgia's president said. "In front of the eyes of mankind, Russian troops are committing ethnic cleansing . . . and I can do nothing about it."

Saakashvili spoke to foreign reporters in Georgia's capital and made an appeal for international help.

Russia sent troops into Georgia after the former Soviet state sent its own forces into the breakaway states South Ossetia and Abkhazia over the past week.

Russia's foreign minister has said that Georgia can "forget about" regaining the two separatist provinces.

The United Nations is estimating about 100,000 civilians have been displaced by the conflict.

Defence Minister Peter MacKay announced Thursday that Canada has pulled out of a planned military exercise with Russia because of the Georgia conflict.

Speaking in Fredericton, MacKay said Canada and the U.S. were no longer participating in the NORAD exercise with Russia called "Vigilant Eagle."

He said he hoped that Russia gets the message.

Canadian Prime Minister Stephen Harper condemned Russia's actions Thursday saying that they demonstrated a "Soviet-era" mentality.

"I am deeply concerned by the notion . . . that Russia has a say or some control over a country outside of its border," Harper said. "This is a very worrisome development."

"I hope Russia will reconsider its actions."

U.S.-Russia tensions

A second U.S. cargo plane delivered aid to Georgia Thursday, increasing tensions between Russia and the Americans.

High-ranking Americans continued to pressure Russia to pull out of Georgia to no avail Thursday.

The relationship between the U.S. and Russia is in jeopardy and could be seriously damaged if the nation doesn't tread carefully in its standoff with Georgia, U.S. Secretary of Defense Robert Gates warned.

Gates told a news conference on Thursday that a joint U.S.-Russia military exercise planned to begin Friday has been cancelled, along with the Canada-U.S.-Russia operation scheduled to start on Aug. 20.

"The Department of Defense will re-examine the entire gamut of our military activities with Russia and will make changes as necessary and appropriate depending on Russian action in the days ahead," Gates said.

He issued a stern warning reminiscent of Cold War declarations between the two nations.

"If Russia does not stop back from its aggressive posture and actions in Georgia the U.S.-Russia relationship could be adversely affected for years to come," Gates said.

However, Gates said he doesn't expect the U.S. to take on a military role in the dispute.

"I don't see any prospect for the use of military force by the United States in this situation," he said when pressed by reporters. "Is that clear enough for you?"

At least one planeload of U.S. humanitarian aid has already arrived in Georgia to help those displaced by fighting. Gates said an assessment is underway to determine how to best distribute the aid to those who need it most.

He described Russia's actions as designed to punish Georgia "for daring to try to integrate with the West" and said it is intended as a warning to other former Soviet members to avoid following Georgia's lead.

No sign of withdrawal

Georgian Prime Minister Lado Gurgenidze said the international community is beginning to unite behind the idea that Russian troops must pull out of Georgia and the break away states in order for talks to resume.

But so far there is no sign that is happening.

"The situation here is indeed tense and we have reports about the movement of the Russian troops almost every hour and there's no discernable pattern other than ...they are not withdrawing just yet," Gurgenidze told Â鶹ӰÊÓnet.

One week ago, Georgian troops entered South Ossetia, which broke away from Georgian rule in 1992 and has acted autonomously ever since. The area has not received international recognition, and many of its residents hold Russian passports.

Russian troops entered the fray one day after the Georgian soldiers moved in. Russia claims 2,000 South Ossetians have been killed by Georgia.

Georgian troops moved into Abkhazia after Abkhazian fighters put a flag on a bridge outside their territory in an attempt to redefine the border. On Wednesday Georgia said its troops had withdrawn from Abkhazia

Gori standoff

Russian tanks rolled into Gori on Wednesday, breaking a ceasefire agreement and raising tensions in the region to a new high.

Gori is 24 kilometres from South Ossetia, the breakaway province at the heart of the dispute.

Russian troops were supposed to withdraw from Gori on Thursday, but the plan seemed to hit a snag in the morning when explosions and gunfire could be heard.

There were reports that the blasts -- which occurred shortly after a tense standoff between Russian and Georgian troops on the edge of the city -- sounded like mortar fire.

"It started out OK but that soon gave way to small sporadic shootings and fires in the provincial town of Gori. The Georgian population and leaders ... are very much concerned about the situation," Zaza Gachechiladze, editor-in-chief of The Messenger, an English-language newspaper in Georgia, told CTV's Canada AM

Georgia claimed more Russian troops were arriving in Gori and that the Russians had seized a military base outside of the city.

"Russia has not taken the right steps and is not following its commitment," Gachechiladze said.

With files from The Canadian Press and The Associated Press