The United States announced Monday it was stepping up efforts to target extremists at home as it seeks to build a broad global coalition against Islamic State militants.

Through a series of new pilot programs in cities across the country, the goal is to create a network of community partnerships "to keep our nation safe," Attorney General Eric Holder said in a statement.

"We have established processes for detecting American extremists who attempt to join terror groups abroad. And we have engaged in extensive outreach to communities here in the US -- so we can work with them to identify threats before they emerge, to disrupt homegrown terrorists, and to apprehend would-be violent extremists," he added.

"But we can -- and we must -- do even more."

The programs -- connecting community representatives, public safety officials, religious leaders and US attorneys -- are being launched by the Justice Department in partnership with the White House, the Department of Homeland Security and the National Counterterrorism Center.

The White House will be hosting a summit dedicated to countering violent extremism in October, Holder said.

"Ultimately, the pilot programs will enable us to develop more effective -- and more inclusive -- ways to help build the more just, secure, and free society that all Americans deserve," he added.

Holder's announcement came as US Secretary of State John Kerry stepped up efforts to forge a broad anti-IS coalition and the world's top diplomats pledged to support Iraq in its fight against the militants by "any means necessary."

Last week, the Central Intelligence Agency said IS militants in Iraq and Syria now have about 20,000 to 31,500 fighters on the ground, much higher than a previous estimate of 10,000.

Among those in Syria are 15,000 foreign fighters including 2,000 Westerners, some of whom have joined IS, a US intelligence official told AFP.

The Pentagon, meanwhile, has said it believes about "a dozen" Americans are fighting with the extremists.

"We must be both innovative and aggressive in countering violent extremism and combating those who would sow intolerance, division, and hate -- not just within our borders, but with our international partners on a global scale," Holder said.

President Barack Obama will lead a UN Security Council session on the threat of foreign fighters in Iraq and Syria on September 24.