U.S. President Barack Obama considers Afghanistan the top overseas military priority and could send as many as three more brigades to the country by the end of the summer, Defence Secretary Robert Gates said Tuesday.

Gates, who appeared before the U.S. Senate armed services committee in Washington, said more infrastructure needs to be put in place in Afghanistan before the additional troops can be sent.

A typical U.S. brigade is made up of about 3,500 military personnel. There are roughly 34,000 American troops already in Afghanistan.

"There is little doubt that our greatest military challenge right now is Afghanistan," the U.S. defence chief said in a prepared opening statement, according to Reuters.

"President Obama has made it clear that the Afghanistan theatre should be our top overseas military priority."

Gates also said international co-ordination of the fight against the Taliban in Afghanistan has been "less than stellar."

U.S. Vice-President Joe Biden delivered the same message Monday on the CBS program Face the Nation, saying that the world hasn't done enough to provide economic, political and military resources to Afghanistan, and the United States and its allies lack a coherent strategy. The result is a country backsliding into Taliban control, Biden said.

More than 2,500 Canadian troops currently have responsibility for one of the more dangerous parts of the country, the region around Kandahar where Taliban militants have been particularly aggressive in attacking foreign troops. British, Dutch and American soldiers are also in the region.

Canada's Conservative government has been calling for other NATO countries to build up their military presence in the troubled nation in order to increase security for the Afghan people and speed rebuilding efforts, more than seven years after the former Taliban regime was toppled.

Last January, Gates complained in an interview with the Los Angeles Times that NATO forces in southern Afghanistan did not know how to properly combat a guerilla insurgency and that could be contributing to rising violence in the country.

He later called Defence Minister Peter MacKay to explain his comments were not directed at Canadian soldiers.

With files from the Associated Press