Baghdad  - Iraqi and US security forces arrested three people suspected of being involved in Tuesday's mortar attacks on Baghdad's Green Zone, the US military said Wednesday.

Mortars fell on Baghdad's Green Zone shortly after US Vice President Joe Biden arrived in the Iraqi capital on a surprise visit Tuesday night.

Soon after Biden met with the head of US forces in Iraq, General Ray Odierno, and US Ambassador Chris Hill, the signal to take cover sounded within the heavily fortified compound, according to a pool report.

The mortars caused no apparent damage, and there were no immediate reports of casualties. Security forces seized three rocket rails believed to have been used in the attack, the statement added.

Biden met with several Iraqi officials, including Foreign Minister Hoshyar Zebari.

Just before the mortars fell, Biden told reporters he had "a personal relationship" with Iraqi leaders, and that he had "won a measure of trust" from them.

He said Iraqi leaders had consulted him in the past about national elections scheduled to take place in January.

"A successful election is the necessary condition for some of the outstanding political issues to be resolved," Biden said. It was his third trip to Iraq this year, following visits in July and January.