BAGHDAD (AP) — A wave of attacks in and around Baghdad on Tuesday killed at least 28 people as Iraqi security forces repelled an attack by the extremist Islamic State group on the country's largest oil refinery, officials said.

The attacks came as Iraqi Prime Minister Haider al-Abadi met with President Barack Obama in Washington and appealed for greater support from the U.S.-led coalition carrying out airstrikes against IS militants who captured a third of both Iraq and neighboring Syria in a blitz last year.

Obama pledged $200 million in U.S. humanitarian aid to Iraq to help those displaced by Islamic State attacks — an offer of assistance that appeared to fall short of the Iraqi prime minister's request for greater military support.

The Islamic State group and other Sunni extremists carry out near-daily attacks targeting Iraq's security forces and the country's Shiite majority.

The deadliest of Tuesday's attacks came at night, when a car bomb explosion at a commercial street killed eight people, including two women, and wounded 21 in eastern Baghdad, police said.

Earlier in the day, seven people were killed when a car bomb exploded in a commercial area in the town of Mahmoudiyah, about 30 kilometers (20 miles) south of Baghdad, a police officer said. The car was parked in a mainly Shiite section of the town near a bakery and went off as people were standing in line to buy bread.

Thirteen civilians were wounded in that attack, the officer said.

In the afternoon, two car bombs went off simultaneously in Baghdad's southeastern outskirts, in al-Wihda area, killing six civilians and wounding 13, another police officer said. One of the cars was parked in a commercial area while the second was in a nearby parking lot.

Another car bomb exploded in a parking lot outside Baghdad's Yarmouk hospital on Tuesday morning, killing four civilians and wounding 10, police said. Three more civilians were killed and eight wounded when a bomb ripped through an outdoor market in Baghdad's northern Sabi al-Bor area, police added.

Medical officials in nearby hospitals confirmed the casualty figures. All officials spoke on condition of anonymity because they were not authorized to release information.

The bombings came a day after attacks in and around Baghdad killed at least 15 civilians.

Also Tuesday, the deputy governor of the northern Salahuddin province said IS militants had used suicide armored car bombs to try to break into the Beiji refinery over the past two days. Ammar Hikmat said security forces repelled the attacks and remain in control of the facility. He said more than 20 militants were killed during the clashes and that several security forces were killed or wounded, without elaborating.

"We call upon the central government to send reinforcements immediately. The soldiers defending the refinery are exhausted and they are in need of any kind of help in order to withstand the attacks," Hikmat said.

Oil Minister Adel Abdul-Mahdi said Monday that Iraqi forces, backed by U.S.-led coalition airstrikes, repelled an IS attack over the weekend on Beiji.

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Associated Press writers Murtada Faraj and Sameer N. Yacoub in Baghdad contributed to this report.