This year's U.S. death toll in Afghanistan the highest

Kabul, Afghanistan (CNN) -- In only eight months, 2010 has become the deadliest year for U.S. troops in Afghanistan, according to a CNN count of Pentagon and NATO figures.

At least 321 troops have died so far in 2010, the highest yearly toll since the conflict began nearly nine years ago. The previous high was last year's 313 American deaths.

The deaths occurred during an increase of U.S. troops in Afghanistan and fierce fighting across the nation, particularly in the southern and eastern regions. Roadside bombs were responsible for many of the combat deaths.

The conflict started after al Qaeda terrorists harbored by the Taliban government in Afghanistan attacked the United States on September 11, 2001. A U.S.-led invasion that started a month later toppled the government.

The fighting persisted, however, as tenacious militants in Afghanistan and Pakistan, near the Afghan border, staged attacks against coalition and Afghan targets.

President Barack Obama, who regards the Afghan war as the central front in the war against terror, said that because of the troop drawdown in Iraq, where the combat mission has now ended, more resources can be freed up for the fight in Afghanistan, including the added troops.

In his Oval Office speech Tuesday night, Obama said the United States is training Afghan security forces and supporting political solutions in the country, and plans to make a transition to Afghan troop leadership in July.

"As with the surge in Iraq, these forces will be in place for a limited time to provide space for the Afghans to build their capacity and secure their own future," he said.