WASHINGTON—The U.S. is providing direct, covert military aid to Kurdish forces struggling to repel well-armed Sunni militants who have seized key parts of northern Iraq, U.S. officials said Monday.

The Central Intelligence Agency has been providing Kurdish fighters with ammunition and small arms, putting the spy agency at the vanguard of what the U.S. hopes will become an international effort. The Pentagon is similarly seeking ways to start its own direct supply efforts.

"We have already provided some arms to the Kurds and the U.S. military is now looking to fulfill some more specific requests that they've had for small arms and ammunition," said one U.S. official.

The CIA declined to comment on the resupply effort.

Kurdish forces are battling the Islamic State militant group, which has swept across Iraq in recent weeks and carved out a quasistate that straddles the border with Syria. They've asked the U.S. government primarily for ammunition that can be used with light and heavy machine guns, along with mortar rounds and hand grenades, officials said.

The U.S. effort to resupply the Kurds is being coordinated with the central government in Baghdad, which has been taking an "unprecedented" lead in funneling military aid to the Kurdish fighters so far, the official said.

The government of Iraq has already provided the Kurdish forces with three planeloads of weapons to battle the Sunni militants, one U.S. official said.

"It's important that the Kurds hold," the official said. "And I think everyone in the United States government understands that."

The U.S. has a consulate in Erbil, the Kurdish capital, and the U.S. Defense Department has set up a special operations center where it has been helping to coordinate the U.S. military operations in the region.

The Pentagon is preparing to begin directly supplying the Kurdish fighters with military aid and is lobbying other nations to join in the effort, officials said.

American officials are appealing to Iraq's neighbors, other Middle Eastern nations, along with France, England, Australia and other key players who may be willing to help with the military resupply effort.

The British military is already helping to drop humanitarian aid to members of the Yazidi religious minority who are trapped by Islamic State forces in mountains near Sinjar.

On Monday, Australia's defense minister said his government is looking at helping the U.S.

"We're not ruling out providing some backup assistance to the Americans as they go in and deal kinetically with this terrorist organization," Defense Minister David Johnston said during a visit to Australia by U.S. Defense Secretary Chuck Hagel.

WSJ