Plans to transfer a top Hezbollah commander who is being held in Baghdad from U.S. to Iraqi custody have been put on hold amid fears in Washington that he will ultimately escape or be freed, a Justice Ministry spokesman said Friday.

Justice Ministry spokesman Haidar al-Saadi said Iraq was waiting for the U.S. to conclude its investigation of Hezbollah detainee Ali Mussa Daqduq. The handover had been expected by the end of the week, but the timing is now 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."

The U.S. military in Iraq has been holding Daqduq since his 2007 capture for allegedly training Iraqi militants to target American soldiers. He was described by a former CIA officer as "the worst of the worst" among Hezbollah's operatives in Iraq.

But Congress and the White House cannot agree where to prosecute Daqduq, and the U.S. military must transfer custody of him and any other detainees to Iraq's government by the end of the year as required under a 2008 security agreement between Washington and Baghdad.

If that happens, 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 little evidence tying them to crimes.

Moreover, Baghdad's recent efforts to improve diplomatic ties with Iran _ which is funding and training Hezbollah in Iraq _ has made U.S. authorities fear that Daqduq will simply be freed if he is turned over.

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 a July 20 Associated Press report 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, Cuba, 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, considered by the U.S. to be a terrorist organization, is a Shiite Muslim group. Al-Qaida is a Sunni organization. The two have had a relationship of convenience at times but 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.

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