Iraqi air strikes killed at least 38 people on Tuesday as security forces held off attacks on a strategic town and an oil refinery, officials and witnesses said.

In the town of Baiji, north of Baghdad, morning air strikes killed at least 19 people and wounded at least 17, officials said, while further raids in the evening killed six more.

The officials said the dead and wounded were civilians, and it was unclear if there were any casualties among the militants who were the target of the strikes.

State television said 19 "terrorists" were killed in the earlier set of Baiji raids.

In the Husseibah area of Anbar province, west of Baghdad, another air strike killed seven militants and six civilians, witnesses said.

Elsewhere in Anbar, security forces and allied tribesmen held off an assault on the strategic town of Haditha, located on the road to provincial capital Ramadi, a police officer said.

Militants also launched a renewed push to seize Iraq's largest oil refinery, which is located near Baiji, but the overnight attack was repelled by security forces, officials said.

The refinery, which filled some 50 percent of Iraq's demand for refined petroleum products in better days, has been the scene of heavy fighting since militants launched a major offensive on June 9, sending jitters through world oil markets.

The militants, led by jihadists from the Islamic State of Iraq and the Levant, or ISIL, have overrun major areas of five provinces and driven to within 100 kilometres (60 miles) of Baghdad.

Security forces performed poorly during the initial onslaught, and are now struggling to hold their ground in the face of the relentless militant push.

US military advisers begin 'limited' mission in Iraq
Washington, United States (AFP) June 24, 2014 - The first of up to 300 US military advisers began their mission in Baghdad Tuesday to help the Iraqi army, but the Pentagon said the American troops were not taking on a combat role.

The primary task of the advisers was to evaluate the state of the Iraqi forces and not to turn the tide against militants from the Islamic State of Iraq and the Levant (ISIL), which have swept across western and northern Iraq, the Pentagon's press secretary said.

"This isn't about rushing to the rescue," Rear Admiral John Kirby told reporters.

"These teams will assess the cohesiveness and readiness of Iraqi security forces ...and examine the most effective and efficient way to introduce follow-on advisers," Kirby said.

The US troops, which included special operations forces, would relay their findings to commanders within "the next two to three weeks."

He did not say how long the advisers would be in place but said: "This is a limited, short-term duration mission."

Two teams of about 40 troops, which were drawn from the US embassy in Baghdad, "have started their new mission," Kirby said.

An additional 90 troops have arrived in Iraq to set up a joint operations center in the Iraqi capital and another 50 are due to deploy in the next few days, he said.

Combined with troops already stationed at the US embassy and others sent to bolster security there, the American military's presence in Iraq was now at about 500 forces, officials said.

After the stunning onslaught of ISIL militants, President Barack Obama announced plans to send the advisers to Baghdad last week while leaving open the possibility of eventual air strikes against the extremists.

The US military, which has deployed an aircraft carrier group to the Gulf, was ready to carry out bombing raids if called upon, Kirby said.

"We remain postured to do that," Kirby said.

But for the moment, the focus was on looking at the Iraqi forces and examining how additional teams of American advisers should be organized, he said.

"This is just the first day of the establishment of these assessment teams."

After swiftly advancing across a swath of territory in the north and west, ISIL forces are trying "to solidify those gains and to continue to threaten Baghdad," Kirby said.

He also said the United States had expanded its surveillance flights over Iraq, with manned and unmanned aircraft, and now was conducting 30 to 35 sorties a day.

In recent days, Iraqi forces have fended off assaults by ISIL at the Baiji oil refinery in the north, the country's largest, and the strategic western town of Haditha.