Militants attacked one of Iraq's largest air bases on Wednesday as the first US teams arrived to assess the Iraqi security forces and decide how to help counter a mounting Sunni insurgency.
Two weeks of advances by militants spearheaded by al Qaeda offshoot the Islamic State in Iraq and the Levant (ISIL) has threatened to rupture the country two and a half years after the withdrawal of US troops.
US Secretary of State John Kerry urged leaders of Iraq's autonomous Kurdish region on Tuesday to stand with Baghdad in the face of the onslaught.
Militants including ISIL and allied Sunni tribes battled Iraqi forces in the town of Yathrib, 90 km north of Baghdad, into the early hours of Wednesday, witnesses and the deputy head of the municipality said. Four militants were killed, they said.
Insurgents have surrounded a massive air base nearby, which was known as "Camp Anaconda" under US occupation, and struck it with mortars.
More than 1,000 people, mainly civilians, have been killed in less than three weeks, the United Nations has said, calling the figure "very much a minimum".
US President Barack Obama has offered up to 300 American advisers to Iraq but held off granting a request by Prime Minister Nuri al-Maliki's government for air strikes.
Rear Admiral John Kirby, the Pentagon press secretary, said late on Tuesday 130 of the advisers had now been deployed, with the initial group sent to establish the operations center included intelligence analysts, logistics experts and special operations forces.
Kirby said about 40 special operations personnel already in the country and assigned to the US Embassy's Office of Security Cooperation had been deployed as part of the first two assessment teams.
About 90 additional troops arrived in Iraq to begin helping establish a Joint Operations Center in Baghdad with Iraqi forces. Another 50 US military personnel working in the region are expected to arrive within the next few days to create four additional assessment teams, Kirby said.
US military personnel also are flying regular manned and unmanned reconnaissance flights over Iraq - about 30 to 35 per day - to give better insight about the situation on the ground and help the assessment teams, he said.
The Baiji refinery, a strategic industrial complex 200 km (120 miles) north of Baghdad, remained a frontline early on Wednesday. State TV showed troop reinforcements flying into the compound by helicopter to fend off the assault.
Local tribal leaders said they were negotiating with both the Shi'ite-led government and Sunni fighters to allow the tribes to run the plant if Iraqi forces withdraw. One government official said Baghdad wanted the tribes to break with ISIL and other Sunni armed factions, and help defend the compound.
Sunni militants are "well positioned" to hold a broad swathe of territory captured in northern and western Iraq if the Baghdad government fails to produce a robust counter-offensive, a senior US intelligence official said on Tuesday.
The official said ISIL risks over-extending itself if it expands too quickly.
The official said ISIL is flush with money and weapons after looting military equipment in Syria and Iraq and raising money through kidnapping, robbery, smuggling and extortion schemes, including the imposition of a "road tax" in Mosul. The official, however, disputed media reports suggesting ISIL's income had soared into the hundreds of millions of dollars, noting its takings in Mosul amounted to "millions" of dollars.
REUTERS