Iraq ? Five U.S. soldiers were killed Monday afternoon in northern Iraq when explosives detonated near their vehicle, the second-deadliest attack on American soldiers this year.

The attack occurred in Mosul, a city that the Iraqi government has dubbed al-Qaida in Iraq’s last major stronghold, and the site of some of the biggest attacks in the country so far this year.

Prime Minister Nouri al-Maliki last week announced a major offensive against Mosul, sending Iraqi troops and tanks to Iraq’s third-largest city. The prime minister promised a “decisive” push through the northwest province.

The U.S. military said Iraqi forces would lead the offensive, which could be a major indicator of the Iraqi military’s ability to stand on its own.

Monday’s attack and a surge in violence outside Baghdad confirmed a reversal of the downward trend in the U.S. death toll, which had fallen for five previous months.

According to icasualties.org, a Web site that tracks casualties in Iraq, 36 soldiers have died so far this year compared with 23 in December. In all, 3,940 American troops have been killed in Iraq since the war began in March 2003.