U.S. and its allies in the coalition fighting the Islamic State in Iraq and Syria said it has killed thousands of militants during its operation to roll back the group.

U.S. Secretary of State John Kerry told reporters Thursday that nearly 2,000 airstrikes have halted the group's momentum, squeezed its finances, killed at least 1,000 fighters and eliminated half of the group's leadership.

A U.S. Central Command official told The Associated Press that a conservative estimate would be about 6,000 militants killed.

At a meeting of anti-Islamic State coalition partners that included British Foreign Secretary Philip Hammond and Iraqi Prime Minister Haider al-Abadi, Kerry acknowledged that the international coalition "can do better" at slowing the number of foreign recruits joining ISIS. He also said the coalition has to challenge the extremists' ideas, which have seized the imagination of Islamic radicals across Europe.

Kerry said the Islamic State group had been "definitively" halted in Iraq, even though the strategic city of Mosul remains in the group's control. Kerry said Iraqi ground troops had retaken 270 square miles of territory and deprived the militants of the use of 200 oil and gas facilities.

But he acknowledged that "we have decided there are things we can do more of," including beefing up efforts to halt foreign fighters, curb the Islamic State's finances and publicly counter the group's social media presence.

U.S. intelligence officials say the pace of foreigners traveling to fight with the Islamic State group has not slowed, and that the group's ideology is spreading.

Kerry's comments come a day after al-Abadi said the U.S. and its partners were not moving fast enough to supply Iraq with weapons.

Al-Abadi said that he asked people for more support at the meeting and did not think his call went unnoticed.

Kerry said the U.S. and its allies were training Iraqi forces "as fast as possible," with a goal of creating 12 new Iraqi brigades. Iraq was also receiving equipment, including 250 mine-resistant armored vehicles and a "very significant number" of M16 assault rifles, Kerry said.

Hammond said the coalition hoped to defeat both the group and its ideology "however long it takes and wherever it leads us."

The Associated Press contributed to this report