Iraq's government said Friday it would delay taking custody of a top Hezbollah commander from the U.S. military after American senators asked the Pentagon "to take whatever steps" necessary to block the transfer for fear he would escape or be released.

The move puts new pressure on Washington decide whether, and where, to prosecute Ali Mussa Daqduq before a year-end deadline when the U.S. military hands over all detainees it's holding in Iraq.

The U.S. military has been holding Daqduq, a Lebanese militant from that country's Shiite Hezbollah guerrilla group, since he was captured in 2007 in the Iraqi Shiite holy city of Karbala.

Dubbed by a former CIA officer as "the worst of the worst," Daqduq is accused of working with Iranian agents to train Shiite militias who targeted American soldiers in Iraq. He was linked to a brazen 2007 raid in which four American soldiers were abducted and killed in the holy Iraqi city of Karbala.

Several days ago, Iraqi Justice Ministry spokesman Haidar al-Saadi said Daqduq would be handed over to Iraqi custody by the end of the week.

But on Friday, al-Saadi said Baghdad would wait until the U.S. has finished an investigation of Daqduq before taking custody of him _ leaving the timing unclear.

"When their investigation ends and he is transferred to the Iraqi side, we will then announce this event," al-Saadi told The Associated Press. "I can't give you an expected date."

Unless the U.S. prosecutes him, the American military must transfer custody of Daqduq and any other detainees to the Iraqi government by Dec. 31 under a 2008 security agreement between Washington and Baghdad. But Congress and the White House have slowed his case by feuding over whether to bring him to the United States for trial or send him to a military court at the Navy base at the Guantanamo Bay, Cuba.

If handed over to the Iraqis, U.S. counterterror officials believe Daqduq will soon be out on the streets.

Numerous high-profile terror suspects have escaped from Iraq's prisons, including some whom investigators said likely had inside help. Additionally, Iraq has released tens of thousands of terror suspects who were captured by U.S. forces during the height of the war because of what Baghdad has described as little evidence tying them to crimes.

Or, U.S. officials worry, Iraq's Shiite-led government will simply free Daqduq, given Baghdad's recent efforts to improve diplomatic ties with Iran, which has funded training for Shiite militias. In a slap to the Obama administration, Iraq's government in 2009 released two of Daqduq's acolytes _ Laith and Qais al-Khazali, who also were implicated in the Karbala attack _ after being lobbied by the Iranian-linked Asaib Ahl al-Haq militia.

In a letter dated Thursday, 20 U.S. senators asked Defense Secretary Leon Panetta "to take whatever steps you can to block Daqduq's transfer to the Iraqi government and out of U.S. custody."

"If he is released from United States custody, there is little doubt that Daqduq will return to the battlefield and resume his terrorist activities against the United States and our interests," the senators wrote in the letter signed by 19 Republicans, including Senate Minority Leader Mitch McConnell of Kentucky and Sen. John McCain of Arizona, top Republican on the Senate Armed Services Committee. Senate Homeland Security Committee chairman Joe Lieberman, an independent senator from Connecticut, also signed the letter.

They were responding to an Associated Press report on Wednesday about Daqduq's imminent transfer.

For years, the U.S. planned to try Daqduq in an American court, but that has stalled as the White House and Congress clashed over how to prosecute suspected terrorists.

Under President George W. Bush, a Republican, U.S. officials planned for military and intelligence officials to question Daqduq, and then let an FBI team start the questioning over from scratch. That way, he could someday be brought to a U.S. court and his statements could be used against him.

But Republican members of the Senate Judiciary Committee in May refused to let Daqduq and other terrorist suspects be brought to the United States for trial.

Instead, the Republicans wanted Daqduq and other suspected terrorists to be prosecuted at the Guantanamo Bay military base, which the Obama administration has tried to close. Lawyers who have reviewed the case concluded that while prosecuting him at Guantanamo Bay is possible, incarcerating him there is not.

That is because Congress authorized military action against al-Qaida and those who carried out the Sept. 11 terrorist attacks. The Supreme Court has relied on that authorization to allow the military to hold al-Qaida suspects at Guantanamo Bay.

Hezbollah, a Lebanese Shiite militant group, is considered by the U.S. to be a terrorist organization but is not known to have significant ties to al-Qaida. The Sept. 11 Commission found no evidence that Hezbollah was aware of or involved in the planning for the attacks on the World Trade Center and Pentagon.

___

Associated Press Writer Hamid Ahmed in Baghdad contributed to this report.