Iraq's Shia-led government has issued an arrest warrant for its Sunni vice-president, accusing him of having conspired to assassinate government officials – making what is an extraordinary charge, only one day after the last US troops had left.

The vice-president, Tariq al-Hashemi, had left Baghdad on Sunday for the semi-autonomous region of Kurdistan, presumably hoping the authorities there will not turn him in; earlier in the day, investigative judges in the capital had banned him from travelling abroad.

The move against Iraq's highest-ranking Sunni elected leader marked a sharp escalation in sectarian tension, raising fear of a resurgence of mass bloodshed. Although many Iraqis had welcomed the American withdrawal, there was also considerable fear violence would worsen afterward. "Iraq is slipping into its worst nightmares now, and Iraqi people will pay a high price because of the struggle among political blocs," predicted one Shia analyst, Kadhum al-Muqdadi, in Baghdad.

In Washington, the White House said the Obama administration had told all the parties involved of its concern at the issuing of the warrant, urging a resolution "through dialogue consistent with rule of law and democratic political process".

Hashemi is a longstanding rival to prime minister Nouri al-Maliki, and the arrest order came two days after the main Sunni political bloc, Iraqiya, suspended its participation in parliament because al-Maliki refused to give up control over key posts. Al-Maliki, a Shia, has made moves in recent months to consolidate his power. Hundreds of former members of the Ba'ath party of Saddam Hussein have been rounded up as security threats, although no proof has been given.

State-run TV ran what it said were confessions by men said to be bodyguards for al-Hashemi. The men said they had killed officials in the health and foreign ministries as well as police officers. "An arrest warrant has been issued against al-Hashemi under the terrorism law, and five judges have signed it," said the interior ministry.

Al-Hashemi, one of two vice-presidents, could not be reached for comment.

After 2003, the Sunni minority first waged an insurgency against the Americans, then became US allies against al-Qaida, but now Sunni relations with the Shia-led national government are fraught. Everyday relations between Sunni and Shia are much better than in the insurgency, when neighbours turned on neighbours and whole sections of Baghdad were expunged of one Muslim sect or the other.

However, this forced segregation, fuelled by extremists, has fundamentally changed Iraq's character. The parliament boycott by Iraqiya, was in response to the government's failure to share more powers, particularly control of security forces, said a Sunni MP. Hamid al-Mutlaq, a member of the bloc.

Iraqiya narrowly won the most seats in last year's election, but its leader Ayad Allawi was outmanoeuvred by al-Maliki, who kept the premier's post after cobbling together support from other Shia parties. The Sunnis feel they are being penalised simply for being Sunnis, while at the other extreme, al-Maliki, for example, spent 24 years in exile and was sentenced to death by Saddam.