Iran, which is helping Iraq in its fight against jihadist-led militants, has sent its foreign minister to Baghdad, as gunmen made a renewed push for the country's main oil refinery.
The United States, whose warplanes have launched more than 90 air strikes against Islamic State (IS) jihadists in Iraq since August 8, has said operations against the group in Syria may also be necessary.
Washington has also ramped up its rhetoric over the grisly IS beheading in Syria of abducted American journalist James Foley, calling it "a terrorist attack against our country".
The toll rose Sunday to more than 35 dead in a string of attacks across Iraq the day before, as officials sought to defuse tensions after 70 people were gunned down at a Sunni mosque by suspected Shi'ite militiamen.
Iraq is struggling to regain ground lost to a major IS-led militant offensive which began in June and quickly overran large areas of five provinces, sweeping security forces aside.
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Iranian Foreign Minister Mohammad Javad Zarif was to meet caretaker premier Nuri al-Maliki, prime minister-designate Haidar al-Abadi and other officials in Baghdad on Sunday.
Tehran has said it is advising Iraq's federal government and its autonomous Kurdish region, which are battling to push back IS-led militants.
However, evidence including the death of an Iranian pilot and the presence of several Iranian Su-25 warplanes points to a more direct military role by Iraq's Shi'ite neighbour to the east.
As Zarif visited Baghdad, security forces backed by air support battled a renewed militant push towards the Baiji refinery, which once accounted for some 50 per cent of Iraq's supplies of refined petroleum products, and has been targeted in the past.
The latest unrest came as the death toll from a string of attacks in and north of Baghdad on Saturday rose to at least 37, a health official said.
Officials have been trying to calm tensions caused by an attack two days ago on a Sunni mosque in Diyala province.
The attack, in which worshippers were sprayed with machine gun fire, killed at least 70 people and wounded 20, officials said.
Dubbed by rights group Amnesty International a "massacre" that Iraqi authorities "must properly investigate", the attack threatens to increase anger among the Sunni Muslim minority with the Shi'ite-led government at a time when the anti-IS drive depends on their cooperation.
Army and police officers said the assault on the Musab bin Omair Mosque in Diyala came after Shi'ite militiamen were killed in clashes, while other sources said it followed a roadside bombing near a militia patrol.
Although some officers blamed IS for the attack, most accounts, including one from an eyewitness, pointed to Shi'ite militiamen.
The government turned to militias to bolster its flagging forces during the IS-led offensive, but in doing so it has encouraged a resurgence of groups involved in brutal sectarian killings in past years.
The United Nations, meanwhile, warned that the Shi'ite Turkmen-majority northern Iraqi town of Amerli is under threat of a "massacre" by jihadists who have besieged it for more than two months.
"The situation of the people in Amerli is desperate and demands immediate action to prevent the possible massacre of its citizens," UN Iraq envoy Nickolay Mladenov said in a statement.
