Iraqi authorities said on Friday a rare attack inside Baghdad's heavily fortified Green Zone this week was carried out by a suicide bomber in a car and may have targeted the country's prime minister.

Reports a suicide bomber was able to penetrate the Green Zone, which houses the U.S. and other embassies as well as parliament and some ministries, raised questions about security just as the remaining American troops leave Iraq.

The attack took place a day before a visit by U.S. Vice President Joe Biden to Iraq's capital.

Baghdad security operations official, Major General Qassim al-Moussawi, said intelligence pointed to an attempt to target Prime Minister Nuri al-Maliki or some other top political leader, but he said the investigation was still ongoing.

It was not clear if anybody other than the attacker was killed in Monday night's blast, which officials initially said was a mortar round. Rockets and mortar rounds occasionally land on U.S. bases and inside the Green Zone.

Moussawi said the bomber was driving a black, four-wheel drive vehicle carrying 20 kg (44 lb) of locally manufactured explosives, which had been placed close to the tank.

Intelligence shows the suicide bomber aimed to enter the building of the parliament and to stay in one of the parking lots until the prime minister ... arrived at the parliament, he said.

He showed a video of a black vehicle approaching a checkpoint outside the parliament building, then backing off, before an explosion outside the building. But he showed little evidence as to how the bomb was to have targeted Maliki.

Violence in Iraq is down sharply since the peak of sectarian slaughter here in 2006-2007, but Sunni Islamists tied to al Qaeda and rival Shi'ite militias, some backed by Iran, still carry out almost daily bombings, attacks and assassinations.

The remaining 12,000 U.S. troops are due to leave in a few weeks, nearly nine years after the invasion that ousted Sunni dictator Saddam Hussein and allowed the country's Shi'ite majority to ascend in power.

Many Iraqis worry that without the buffer of a U.S. presence, sectarian tensions could rise again. The admission that a suicide bomber entered the Green Zone as the U.S. hands over security may fuel tensions among the country's fragile power-sharing coalition of Shi'ite, Sunni and Kurdish political blocs.

Supporters of Parliament Speaker Osama al-Nujaifi, a Sunni, have already said he could have been the target of an attempted assassination. He is a political rival of Maliki, a Shi'ite leader.

(Reporting by Waleed Ibrahim; Editing by Patrick Markey and Alessandra Rizzo)