The United States is on the verge of suspending talks with Russia on trying to implement a ceasefire agreement to end the violence in Syria, U.S. Secretary of State John Kerry said Thursday.

“We are on the verge of suspending the discussion because it is irrational in the context of the kind of bombing taking place to be sitting there trying to take things seriously,” Kerry told an audience in Washington.

“It is one of those moments where we are going to have to pursue other alternatives,” he added.

On the other hand U.S.-led airstrikes and its allies have reportedly killed 18 ISIS “leaders” in the last 30 days, 13 of them in Mosul, the group’s de facto Iraqi capital, a U.S. military spokesman said on Thursday.

Earlier this week the Pentagon announced that the U.S. would send around 600 new troops to Iraq to assist Iraqi forces in the battle to retake Mosul from ISIS militants, who control parts of Iraq and Syria.

Colonel John Dorrian, a spokesman for U.S. forces in Iraq and Syria, told a Pentagon briefing that many of those targeted were military commanders, propagandists and those facilitating foreign recruits into territory controlled by the ultra-hardline terror group.

“By taking these individuals off the battlefield, it creates some really disruptive effects to enemy command and control,” Dorrian said.

Dorrian said there are between 3,000 and 4,500 ISIS left in Mosul and while new fighters are not able to enter the city in large convoys, they still continue to move in small formations.

The United States has 4,565 troops in Iraq as part of a U.S.-led coalition providing extensive air support, training and advice to the Iraqi military, which collapsed in 2014 in the face of ISIS’ territorial gains and lightning advance toward the capital, Baghdad.

Source: Reuters