Millions of volunteers have flocked to join Iraq's beleaguered security forces as a Sunni-led insurgency continues to overwhelm large parts of the country.

Among those stranded in territory taken at lightning speed by the Islamist fighters, more than 1,000 Chinese workers are reportedly hemmed in at a construction site in the conflict zone in the country's north.

Iraq's Deputy Prime Minister Hussain al-Shahristani said Tuesday during an interview on state television that some 2 million volunteer militia fighters have been recruited, after a call to arms from top Shiite cleric Grand Ayatollah Ali al-Sistani.

Those with military experience have already been deployed to battlefields in Mosul and Tikrit to fight militants spearheaded by the Islamic State in Iraq and Syria (ISIS). The government is also stepping up training for the new recruits, Shahristani said.

Meanwhile, Iraqi Shiite volunteers, who had been fighting in neighboring Syria, have been heading home to battle ISIS, the Syrian Observatory for Human Rights said Tuesday.

Thousands of Iraqi Shiites had flocked to Syria to fight alongside Syrian President Bashar al-Assad's forces against mainly Sunni rebels.

State television said Iraqi security forces repelled attacks on three neighborhoods overnight in Baquba, capital of Diyala Province, which neighbors Baghdad.

The attack on Baquba marked the closest that fighting has come to Baghdad after jihadists said they intend to march on Baghdad and the southern Shiite holy city of Karbala as part of their offensive.

State TV reported Tuesday that government forces had killed more than 200 members of ISIS near Mosul. A security force commander also said his troops are fighting an ISIS group near Tal Afar, a strategic Shiite-majority town in northern Iraq that lies along a crucial corridor to Syria, and he expects to regain control of the region soon.

According to Nuriddin Qabalan, deputy provincial council chief, militants control most of Tal Afar and the surrounding area with soldiers, policemen and residents holding on to parts of the airport.

China, which has a sizable business presence in the war-torn country's reconstruction and resource exploitation, has also been affected.

A Chinese businessman in Baghdad told the Global Times Tuesday that more than 1,000 Chinese workers from China Machinery Engineering Corp (CMEC) who are working on a power plant project in Samarra, Salaheddin Province, are stranded due to battles between government forces and militants.

The plant contract was signed in December 2012 and is worth more than 1.85 billion yuan ($292 million), according to CMEC's website.

Representatives from CMEC's Baghdad office tried to board a helicopter to the site in a rescue mission Tuesday but were stopped by Iraqi military personnel at Baghdad airport, who cited security concerns, the source said.

The wife of one of the stranded workers wrote on her Sina Weibo account that she had lost contact with her husband since Friday and called for support from the media.

"Local translators and police have all run away. The Chinese workers were left on their own with little language capability and limited food and water," wrote the desperate wife, who went by the Internet name Christinali09. She had not replied to an interview request from the Global Times as of press time Tuesday.

The Chinese Embassy to Iraq has yet to confirm the incident with the Global Times as of press time.

The situation in Iraq has prompted US and Iranian officials to discuss the crisis in Vienna on the sidelines of separate negotiations about the Iranian nuclear program, but both ruled out military cooperation.

US President Barack Obama announced that 275 military personnel were being deployed to Iraq to help protect the embassy in Baghdad and assist US nationals there, noting that they were "equipped for combat."

Washington has already deployed an aircraft carrier to the Gulf, but Obama has ruled out a return to combat in Iraq for US soldiers, who left the country at the end of 2011.

"Despite the quick invasion of ISIS, the Iraqi government should be capable of controlling the situation on its own. After all, it took years for the US to pull out from Iraq and the US will not want to get sucked into another Iraq war," said Yin Gang, a Middle East expert from the Chinese Academy of Social Sciences.

Washington has pushed Iraqi Prime Minister Nouri al-Maliki to reach out to Sunnis to rebuild national unity as the only way of preventing the disintegration of Iraq.

However, Hassan Suneid, a close Maliki ally, on Tuesday told a TV channel controlled by Maliki's party that the governing Shiite National Alliance should boycott all work with the largest Sunni political bloc, Mutahidoon.

Baghdad Tuesday also blamed Saudi Arabia in a severely worded statement, which said it has promoted "genocide" in Iraq by backing Sunni militants.

Saudi Arabia funds and supplies Sunni rebels in Syria but denies it is behind ISIS.

Agencies contributed to this story