BAGHDAD — A roadside bomb killed four Shiite pilgrims and wounded 15 south of Baghdad today in at least the third fatal attack on people traveling to one of their sect’s most sacred gatherings, officials said.

The death toll rose from 40 to 56 from a suicide bombing yesterday — one of Iraq’s deadliest attacks this year.

In eastern Baghdad, another roadside bombing wounded three pilgrims. A second bomb that went off a few minutes later about 70 yards away wounded a traffic policeman riding to the scene on his motorcycle.

The suicide bomber went after travelers enjoying tea and refreshments yesterday in a tent near Iskandariyah, 30 miles south of Baghdad. The blast killed at least 56 people and wounded 68, according to police and a Babil health department director, Dr. Mahmoud Abdul-Rida, driving the total number of pilgrims to 63 in two days.

Extremists had attacked another group of pilgrims with guns and grenades hours earlier in the predominantly Sunni Baghdad neighborhood of Dora, killing three and wounding 49, an Iraqi military spokesman, Major General Qassim Atta, said. He said the extremists fired from a mosque at the pilgrims and that a counterattack killed five of them, while two were captured.

American and Iraqi forces have increased the number of checkpoints and imposed car bans and other measures in major Shiite cities to protect the worshippers traveling to Karbala, the burial site of the grandson of the Prophet Muhammad, Imam Hussein, who died in a seventh-century battle nearby and became one of Shiite Islam’s most revered figures. Ceremonies will culminate in Karbala Wednesday to commemorate the end of the 40-day mourning period following the anniversary of his death

Major Shiite events have frequently been targeted in the past by suspected Sunni insurgents led by Al Qaeda in Iraq in their drive to stoke sectarian violence.

Recent commemorations — including the Ashoura festival in mid-January to mark Imam Hussein’s death — have passed without major bloodshed amid an overall decline in violence across Iraq.

But the pilgrims who walk for days to reach the shrine of Hussein are vulnerable despite the increased security.

Suicide attacks and car bombings are frequently blamed on Al Qaeda in Iraq, but the American commander of the 4th Brigade, 3rd Infantry Division, Colonel Tom James, which is responsible for the area around yesterday’s attack, said it was too early to say who was behind that bombing.

Sunni leaders denounced the bombing, with the bloc of a hard-line politician, Adnan al-Dulaimi, blaming it on foreigners “aiming to create sectarian strife and to destabilize the country.”