The military has upgraded to "significant" the level of risk it faces in defending the nation, and attributes the higher level to the additional stress placed on the Armed Forces by ongoing wars in Iraq and Afghanistan.

The Pentagon assessment, sent Monday by Joint Chiefs of Staff Chairman Gen. Peter Pace to Congress, measures the strains on the military's ability to carry out national military strategy. The risk is an increase from the "moderate" level assessed in the last biannual report.

The "significant" level means Defense Secretary Robert Gates will have to lay out to Congress "a risk mitigation assessment" detailing what the Pentagon is doing to lower the risk and how quickly the military can respond to another crisis.

A U.S. military official said the assessment found the military is currently less capable of taking on another challenge because of the stress on the force caused by repeated deployments to Iraq and Afghanistan.

If another conflict broke out involving the United States, the military could prevail but it would likely be more difficult because the Armed Forces are not as well-trained and equipped as they would be if the Pentagon were not already engaged in two wars.

U.S. military officials say the challenges that were likely mentioned in the risk assessment sent to Congress include increased violence in Iraq or Afghanistan, threats posed by Al Qaeda and other terrorist networks, Iranian sponsorship of terrorism in Lebanon and Iraq, China's continuing military buildup and the weapons threat from North Korea.

Pace has said on several occasions that potential U.S. enemies should not be deceived into thinking the military is not fully capable of prevailing in another conflict simply because it is engaged in two prolonged wars. However, he's also said any new battle might take longer to win and involve more American casualties.

FOX News' Molly Henneberg and Nick Simeone contributed to this report.