By Waleed Ibrahim

BAGHDAD (Reuters) - A roadside bomb killed five Iraqi policemen Thursday in the once turbulent but recently secure western city of Falluja, police said, a day after at least 72 people died in a market bombing in Baghdad.

A string of attacks has cast doubt on the ability of Iraqi forces to keep the lid on a stubborn insurgency after U.S. combat troops withdraw from towns and cities by June 30.

"The political nature of these attacks is becoming clear. They are an attempt to delay or suspend the withdrawal of U.S. forces from Iraqi urban centers according to the timetable," Vice President Tareq al-Hashimi said in a statement.

In another bombing Thursday, at least two people died and 30 were wounded when an explosion tore through a crowded bus terminal in the south of the capital, a hospital source said.

Police in Falluja in Anbar province, once the heartland of the rebellion against U.S. troops and government forces, said the blast there destroyed a police vehicle and killed all five policemen inside, including a lieutenant.

It came hours after an explosion tore through a busy second-hand market in eastern Baghdad's Sadr City slum, killing 72 people [ID:nLO706572] just four days after U.S. forces handed security of the Shi'ite Muslim area to Iraqi troops and police.

Saturday, a massive truck bomb killed 73 people near the northern city of Kirkuk. That and the Sadr City market bombing were the bloodiest attacks in the country for more than a year.

Violence has dropped sharply across Iraq over the past year, but militants including Sunni Islamist al Qaeda continue to launch car and suicide bombings aimed at undermining the Shi'ite Muslim-led government and reigniting sectarian conflict.

Prime Minister Nuri al-Maliki, a member of Iraq's Shi'ite majority, has urged Iraqis not to lose heart if insurgents take advantage of the U.S. military drawdown to step up attacks.

Analysts say attacks are also likely to intensify ahead of a parliamentary election in January that will be a test of whether the country's feuding factions can live together after years of sectarian slaughter unleashed by the 2003 U.S. invasion.

(Additional reporting by Fadel Al-Badrani in Falluja; Writing by Daniel Wallis; Editing by Samia Nakhoul)