The Iraqi prime minister, Nuri al-Maliki, showed an unexpected streak of stubbornness yesterday in his stand-off with the US over the Blackwater shootings, insisting that action had to be taken against the private security firm.

In an interview with the Associated Press, Mr Maliki, who is in New York for the United Nations general assembly, said Blackwater posed "a serious challenge to the sovereignty of Iraq and cannot be accepted".

His comments were at odds with a briefing of journalists by an Iraqi official in Baghdad who said the expulsion of Blackwater, which has 1,000 staff in the country and provides protection for the US ambassador and other US diplomats, would leave a security vacuum.

Blackwater guards are alleged to have shot 11 civilians in Baghdad last Sunday while protecting a US diplomatic convoy. Blackwater said they were returning fire, while the Iraqi government insisted they opened fire first.

Although the Iraqi government's immediate response was to order the expulsion of Blackwater, the guards are still in Iraq and resumed work on Friday. The US secretary of state, Condoleezza Rice, has pleaded for them to remain and the row has turned into a test of Iraqi autonomy.

Mr Maliki, whose control over an unwieldy coalition government is shaky, is under pressure to punish Blackwater, which has been involved in a series of controversial shootings in Iraq.

But an Iraqi government spokesman on security, Tahseen al-Sheikhly, told a news conference in Baghdad that Blackwater and other western security firms were doing a good job protecting diplomats.

"If we drive out or expel this company immediately there will be a security vacuum that will demand pulling some troops that work in the field so that we can protect these institutes," he said. "This will create a security imbalance."

Although security firms are not subject to Iraqi law, Mr Sheikhly said Iraqi courts should deal with any crime.

A new point of tension emerged yesterday over the US military forces' arrest of an Iranian on Thursday. Mr Maliki, a Shia Muslim who has a good relationship with Tehran, said the man had been invited to Iraq. "The government of Iraq is an elected one and sovereign. When it gives a visa it is responsible for the visa. We consider the arrest ... of this individual who holds an Iraqi visa and a [valid] passport to be unacceptable."

© 2007 The Guardian