美国承认为击败伊斯兰国正艰苦努力

The United States says it is stepping up airstrikes against Islamic State targets in Syria and Iraq, as top U.S. military leaders acknowledged to lawmakers Tuesday that the U.S is struggling to combat the insurgent group.

"No one is satisfied with our progress to date," Marine General Joseph Dunford, the chairman of the Joint Chiefs of Staff, told a hearing on the American military strategy in the Middle East.

Defense Secretary Ash Carter told the Senate Armed Services Committee he was "disappointed" in the failure of a $500 million U.S. effort to train moderate rebel forces in Syria to fight against the Islamic State. The United States abandoned the program earlier this month after only a few soldiers had been trained.

In response, Carter said the United States has intensified its aerial campaign against the Islamic State in hopes of shrinking its hold on Raqqa in northern Syria, the headquarters of its operations.