A bloc comprising Iraq's biggest Shi'ite parties is close to nominating a prime minister, the deputy speaker of parliament said on Monday, suggesting Nuri al-Maliki would have to step aside.

Haider al-Abadi's comments in a tweet came after police sources said special forces and Shi'ite militias loyal to Maliki had been deployed in strategic areas of Baghdad after he made a defiant speech on television suggesting he would not cave in to pressure to drop his bid for a third term.

Abadi is one of the people that has been mentioned as a possible successor to Maliki.

Earlier, Maliki had indicated that he will not drop his bid for a third term and accused the president of violating the constitution in a tough televised speech likely to deepen political tensions as a Sunni insurgency rages.

Special forces loyal to Maliki were deployed on Sunday night in strategic areas of Baghdad, police said.

Several police sources also said the forces had taken up positions at key entrances to the sprawling capital.

Maliki, seen as an authoritarian and sectarian leader, has defied calls by Sunnis, Kurds, some fellow Shi'ites and regional power broker Iran to step aside for a less polarising figure who can unite Iraqis against Islamic State militants.

Deputy Assistant Secretary of State for Near Eastern Affairs Brett McGurk Tweeted Sunday evening that the U.S. supports Iraq's president.

Fully support President of #Iraq Fuad Masum as guarantor of the Constitution and a PM nominee who can build a national consensus. — Brett McGurk (@brett_mcgurk) August 10, 2014

CNN's Jim Sciutto says this places the U.S. at odds with Maliki.

Iraqi's took to soci are some reactions and reports regarding Maliki's speech:

Maliki is basically openly declaring war on political system in Iraq - this is an even more dangerous turn of events — Mina Al-Oraibi (@AlOraibi) August 10, 2014

It's all going down now. His party against him, his bloc against him, all politicians against him, but he is entrenched. Fight for power — Sajad Jiyad سجاد (@SajadJiyad) August 10, 2014

Pivotal moment in #Iraq: #Maliki is clinging to power by guarding his palace with private security, as talk of multi-party coup increases. — HRH Robbie Travers (@RobbieTravers) August 10, 2014

Shia militias, Iraqi security forces led by Shia commanders have encircled Green Zone --@mtawfeeqCNN — Robert Caruso (@robertcaruso) August 10, 2014

A coup in Baghdad? Maliki announces he won't resign and orders security forces on alert. So much for bringing democracy to Iraq. — Liz Sly (@LizSly) August 10, 2014

#BreakingNews Eyewitnesses: More army enforcements and hundreds of soldiers are now being deployed across all of #Baghdad. #Iraq #ISIS — Rami (@RamiAlLolah) August 10, 2014

Kurds saving Azadis, fighting IS with US air cover. Now a Kurd is the guarantor of Iraq's constitution, and the only hope for nat consensus. — Michael Weiss (@michaeldweiss) August 10, 2014

(Reporting by Ahmed Rasheed; Editing by Sandra Maler; writing by Michael Georgy)