A suicide bomber detonated his vehicle Tuesday at a checkpoint near the western city of Ramadi, killing two US Marines and wounding three others, the military said. One civilian also died and two dozen other people were wounded in the 7:30 a.m. blast, the latest in a string of recent attacks that appear to have been carried out by al-Qaida in Iraq. Ramadi is the capital of the former Sunni insurgent stronghold of Anbar province, but has been relatively peaceful since local Sunni tribal leaders joined forces with the US military against the terror movement. Iraqi police said the attacker drove a small water tanker packed with explosives to the checkpoint and detonated them when he reached the guards. In Diyala province, a woman detonated an explosive vest at the entrance to an Iraqi police station in Jalawla, killing five policemen and one Kurdish security guard, police said. A dozen other people were wounded. It was the second suicide attack by a woman in as many days in Diyala, which has been a flashpoint in the ongoing battle against al-Qaida. A young woman blew herself up Monday at the headquarters of a group of US-allied Sunni fighters, killing three people and wounding three others, the US military said. Al-Qaida has been regrouping after suffering a devastating blow last year when thousands of Sunni tribesmen turned against them. Last weekend, al-Qaida announced a one-month offensive against US troops and Sunnis who have joined forces with the Americans. Osama bin Laden's deputy, Ayman al-Zawahri, said in an audiotape posted Tuesday on the Internet that al-Qaida-led insurgents in Iraq were the "primary force" in battling the Americans and "challenging Iranian ambitions" in this country. The renewed activity by al-Qaida, a Sunni extremist group, has occurred while attention has focused on the Iraqi government crackdown against Shiite militias, chiefly the Mahdi Army of anti-American cleric Muqtada al-Sadr. Last month Prime Minister Nouri al-Maliki, a Shiite, launched an offensive against Shiite militias in the southern city of Basra. The fighting quickly spread to Sadr City, the Mahdi Army's Baghdad stronghold, where militants fired rockets at the US-protected Green Zone. In the latest fighting, an unmanned US drone killed two gunmen in Sadr City, the US military said Tuesday. US Apache attack helicopters and armed Predator drones have been launching daily strikes against militiamen in Sadr City, a sprawling district of 2.5 million people. Also Tuesday, the US command announced that an American Marine was killed and another wounded the day before when a roadside bomb struck a US convoy in Basra. US and British forces have been assisting Iraqi forces in Basra since their offensive bogged down due to poor planning, widespread desertions and fierce resistance by the militias. Also in Basra, a senior aide to Iraq's top Shiite cleric Grand Ayatollah Ali al-Sistani died on Tuesday, a week after being seriously injured in an assassination attempt. Two other al-Sistani representatives were injured in separate ambushes. The attacks came just days after a top aide of al-Sadr was killed in Najaf - part of the ongoing Shiite power struggle. The crackdown had led to fears that al-Sadr, who is believed to be in Iran, may declare full-scale war on the US-backed government. On Monday, top Sadrists warned that open warfare was a "strong possibility" and complained that the government appeared uninterested in a peaceful settlement.