Two leading members of Iraq's largest and most powerful Sunni tribe have warned of imminent sectarian chaos in the wake of the US withdrawal, claiming that the government of prime minister Nouri al-Maliki is promoting an anti-Sunni agenda.

The sheikhs, leaders of the highly influential Duleimi tribe, both insist that Sunnis have been increasingly marginalised over the past year to the point where they now have little input into affairs of state in post-US Iraq.

Their warnings come as Iraq's vice-president, Tariq al-Hashimi, defended himself over claims in an arrest warrant issued for him that he had used his guards to act as hit squads to target political rivals and had ordered a recent car bombing near the Iraqi parliament.

The dramatic allegations against one of the highest ranking Sunni figures in government have sharply raised the stakes in Iraq. The crisis risks unravelling a fragile power-sharing deal among Shia, Sunni and Kurdish blocs that have struggled to overcome tensions since sectarian slaughter drove Iraq to the edge of civil war in the years after Saddam Hussein fell in 2003.

Senior Iraqi politicians have been holding talks with Maliki and other leaders to contain the dispute.

On Monday the president, Jalal Talabani, a Kurd, and Masoud Barzani, president of the Kurdistan regional government, called for dialogue among the different parties.

"We call for a national political conference urgently to prevent the political process from collapsing and exposing the country to uncalled consequences," Barzani said in a statement.

The unravelling domestic scene is in stark contrast to the portrait painted by US commanders of a representative government that has found its feet after almost nine years of war.

The claims about Hashimi, made on state television, which aired the alleged confessions of three of his guards, have inflamed already high tensions between Sunni politicians and the Shia-led government of Maliki, which last week ordered a second prominent Sunni figure, deputy prime minister Saleh al-Mutlaq, to stay away from parliament.

The Sunni-dominated Iraqiya bloc, which has 91 seats in the 325-seat parliament, has flagged a boycott from the legislature by many of its members. Three Sunni provinces have made unilateral declarations of autonomy.

Sheikh Ali Hatem Sleiman al-Duleimi, whose Baghdad compound was recently confiscated after he publicly insulted Iran's most senior cleric, Ayatollah Ali Khamenei, accused Maliki of moving swiftly to consolidate sectarian rule in a vacuum left by the US military, which left Iraq last Thursday. "There is no democracy here," he said in Ramadi. "There is chaos. Parties rule by sect. Corruption is rampant and so is sectarianism. But more dangerous than anything else is that Maliki is trying to establish a new autocracy."

Anbar, a western Iraqi province bordering eastern Syria, is one of the most strategic locations in the region. Almost exclusively Sunni, it has been at odds with Baghdad ever since the ousting of Hussein.

Anbar's power base was rooted in Saddam's regime and the loss of such access to power was a key driver of a potent al-Qa'ida-led insurgency that bogged down the US military and accounted for around one third of its casualties.

Now the province seems to again be on a war footing, with jihad websites making a call to arms in recent weeks, which has alarmed Baghdad – and neighbouring Damascus, where a Sunni-led insurgency against the Allawite regime of the Assad family is taking shape.

A second sheikh, the elder and leader of the Duleimi tribe, Sheikh Majid Sleiman, said the deteriorating situation in Syria and the increasingly sect-based feuds in Iraq were combining to imperil the region.

"But if our brothers [in Syria] seek our help we cannot abandon them. The people here are energised to go there to help. If the people want to seek shelter here we will not be late in helping them."