BAGHDAD - Intelligence officials say foreign fighters have been slipping back into Iraq in larger numbers recently and may have been behind some devastating attacks this year, reviving a threat the U.S. military believed had been almost eradicated.

It is impossible to ascertain actual numbers of insurgents entering the country. But one Middle Eastern intelligence official estimated recently that 250 came in October.

U.S. officials say the figure is far lower, but have acknowledged an increase since August.

At the same time, Iraqi officials say there has been a surge in financial aid to al-Qaeda's front group in Iraq as the U.S. military prepares to leave by the end of 2011. They say it reflects fears by Arab states over the growing influence of Iran's Shiite-led government over Iraq and its Shiite-dominated government.

On Sunday, Maj. Gen. Qassim al-Moussawi, a security official, said Iraqi forces were searching for six foreign fighters suspected of involvement in the Oct. 31 siege of a church that left 68 people dead and drew outrage. They are also suspected in two summertime attacks on an Iraqi army headquarters in central Baghdad that killed 73 people.

"All who committed these attacks are [non-Iraqi] Arabs," he said. "This indicates the failure of al-Qaeda leaders to recruit Iraqis to carry out suicide attacks."

Moussawi said that five of the six suspects were hiding in two Sunni Muslim-dominated provinces bordering Syria and that one had fled to Syria.

Four of the church bombers were from Libya and Syria and carried fake ID cards that identified them as mutes to avoid talking in foreign accents to checkpoint guards, Iraqi Deputy Interior Minister Ahmed Abu Raghef said.

Army Col. Barry Johnson, a spokesman for U.S. forces in Iraq, said the military noticed a slight increase in foreign fighters starting in August, but he would not say how many. He said the number remained far lower than when insurgents were rushing in between 2005 and 2007.

Last year, U.S. counterterrorism officials said the number of foreigners heading to Iraq had trickled from hundreds to "tens" in what they described as a severely weakened al-Qaeda in Iraq.

But a Mideast counterterrorism official, speaking on condition of anonymity, said 250 foreign fighters entered Iraq in October. He said they came through the Syrian city of Homs, a hub for Syrian Muslim fundamentalists run by Tunisians and Algerians. Other fighters have come from Pakistan, Saudi Arabia, Libya, and Yemen.

The official said tens of millions of foreign dollars annually was funding the Iraqi insurgency, which has received $5 billion in aid since 2007. The money comes from al-Qaeda leaders, Muslims who want the United States to leave, and Arabs eager for Sunni Muslims to regain power in Shiite-majority Iraq.