A U.S. airstrike near Baghdad on Monday marked a new phase in the fight against ISIS.

The airstrike southwest of the city appears to be the closest the U.S. airstrikes have come to the capital of Iraq since the start of the campaign against ISIS, a senior U.S. military official told CNN. And U.S. Central Command said in a statement that it was the first strike as part of "expanded efforts" to help Iraqi forces on the offensive against ISIS.

Monday's airstrike destroyed an ISIS fighting position that had been firing at Iraqi forces, Central Command said.

It occurred about 35 km (22 miles) southwest of Baghdad, another U.S. official said.

The United States began targeted airstrikes against ISIS in Iraq last month to protect American personnel and support humanitarian missions. Last week, U.S. President Barack Obama said new airstrikes would aim to help Iraqi forces on the offensive against the Islamist militants.

Obama also said airstrikes would include ISIS targets in Syria. And last week he also asked Congress for authorization to train and equip moderate Syrian rebels.

The authority comes under Title 10 of the U.S. code, which deals with military powers, and Congress could vote on granting it this week. Approval also would allow the United States to accept money from other countries for backing the Syrian opposition forces.

A senior administration official told reporters Monday that Obama has been making calls to Democratic and Republican members of Congress, asking them to pass the authorization.

U.S. Secretary of State John Kerry courted Middle Eastern leaders over the weekend to join a coalition in the fight against the Islamist militant group, which calls itself the Islamic State.

More than two dozen nations, the Arab League, the European Union and United Nations met in the French capital Monday, calling ISIS a threat to the international community and agreeing to "ensure that the culprits are brought to justice."

The United States has conducted more than 150 airstrikes in Iraq against ISIS, and Kerry has said nearly 40 nations have agreed to contribute to the fight against the militants. But it remains unclear which countries are on that list and the precise roles they'll play.

(CNN)