The Iraqi prime minister has rejected the creation of a government of "national salvation" despite pleas from the US and Britain to form a more inclusive administration to fight a growing insurgency of jihadists and disaffected Sunnis.

Nouri al-Maliki, who has been criticised for putting Shia sectarian interests ahead of national goals, struck an uncompromising tone in his weekly national address.

"The call to form a national emergency government is a coup against the constitution and the political process," he said. "It is an attempt by those who are against the constitution to eliminate the young democratic process and steal the votes of the voters."

Maliki's electoral bloc won the most seats by far in April's parliamentary elections with 92 seats, nearly three times as many as the next biggest party, and he won 720,000 personal votes. But Maliki fell short of a majority in Iraq's 328-seat council of representatives, and has had to court the support of rivals in order to form a government. Parliament is due to begin its first session by 1 July.

Maliki gave only a vague call for "all political forces to reconcile". His remarks came a day after the first of up to 300 US military advisers arrived in Iraq. The Pentagon has ruled out a combat role, and the advisers' main job is to evaluate Iraqi forces and not to turn the tide against the militants, the Pentagon spokesman Rear Admiral John Kirby said.

The US-trained army has buckled under the jihadist offensive in northern Iraq, giving up cities such as Mosul and Tikrit. Kirby said the US had expanded its surveillance flights over Iraq, with manned and unmanned aircraft, and was conducting 30 to 35 sorties daily.

In the latest fighting, insurgents surrounded a massive airbase, known as Camp Anaconda under US occupation, and struck it with mortars. Witnesses said the airbase near the town of Yathrib, about 60 miles (90km) north of Baghdad, had been surrounded on three sides.

Elsewhere, the Islamic State in Iraq and the Levant (Isis) is consolidating its hold on the Iraq-Syria border. The al-Nusra Front, al-Qaida's affiliate in Syria, has pledged loyalty to Isis, which it battled for months in factional fighting. The move clears the way for a joint push to take control of both sides of the frontier between eastern Syria and western Iraq, removing a threat to Isis.

"They are rivals, but both groups are jihadists and extremists," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.

At least 13 people were killed and 25 were wounded when a suicide bomber detonated a device at an outdoor market south of Baghdad. No group claimed responsibility for the attack, but it bore the hallmarks of Sunni militants who have long targeted Shia civilians.

Iraq's predicament is to be discussed at a Nato meeting in Brussels. John Kerry, the US secretary of state, who urged Maliki to form a broad government when he was in Baghdad, discussed Iraq with Catherine Ashton, the EU's foreign policy chief, and other European foreign ministers.

Angela Merkel, the German chancellor, said Iraq should be pressured to usher in a new inclusive government. In a speech to parliament, she said: "We need a government in Iraq that embraces all parts of the population. For years this has not happened and because of this the pressure needs to be raised."