It may take up to two years for forces in the U.S.-led coalition to completely oust the Islamic State group from Iraq, the British foreign minister said Thursday ahead of a meeting with 21 coalition members to discuss ways of expanding the international operation against the brutal militants.

"This isn't going to be done in three months or six months. It's going to take a year, two years to push ISIL [also known as ISIS] back out of Iraq, but we are doing the things that need to be done in order to turn the tide," Foreign Secretary Philip Hammond told Sky News.

The coalition campaign launched more than 1,000 airstrikes and has killed more than 6,000 fighters in Iraq and Syria in the nearly five months since President Barack Obama pledged to “degrade and ultimately destroy” the militant group, according to CNN.

The coalition has "stopped ISIL's advance in Iraq, having negated their resources, their capacity to move foreign fighters, to a significant degree, and changed their operations as a result of what we've been able to do," U.S. Secretary of State John Kerry said before Thursday’s meeting. “We still have a lot of work to do."

U.S. intelligence officials have said the group, which aims to establish an Islamist caliphate, could command a force of up to 31,000 if needed but that the number of active fighters is between 9,000 and 18,000. That said, ISIS has crafted a strong recruitment campaign and some have estimated that new fighters, most of them foreign, join the group every day.

More than 60 countries have joined the coalition, with varying degrees of participation. So far the coalition has focused on an air campaigns and providing assistance to local forces on the ground like the Iraqi Armed Forces, Kurdish fighters and moderate rebel groups in Syria.

The U.S. and some coalition members have sent thousands of military trainers to help Iraqi forces in their fight against ISIS. However, progress has been slow.

"It will be months yet before they are ready to start significant combat operations,” Hammond told BBC radio, as reported by the New York Times.

Kurdish forces have been in almost constant battle with the militants in Syria’s Kobani and Iraq’s Anbar province. On Wednesday, Iraqi peshmerga forces were able to push ISIS fighters out of a 500-square-kilometer (200-square-mile) swath of territory in the province and break a supply route connecting militant strongholds in the west with Mosul, according to the New York Times.

However, a peshmerga commander warned Thursday that if the Kurds did not get reinforcements from the coalition, the ISIS threat would continue to grow.

"We need continuous support and training,” Lieutenant General Jabar Manda told Sky News. “We are in need of ammunition. We need new weapons to win this war."