Antigovernment militants launched an attack Wednesday on Iraq's largest oil refinery, as Prime Minister
Meanwhile, officials in India confirmed that 40 Indian construction workers had been abducted near the northern Iraqi city of Mosul, which was overrun last week by Islamist rebels.
The Indian government has received no ransom demands and no information about the workers' whereabouts, officials in India said.
Dozens of Turkish citizens, including the consular staff in Mosul, also reportedly remain in insurgent hands after having been detained in northern Iraq.
In Washington, the Obama administration is debating whether to expand military assistance to the besieged Iraqi government, including launching airstrikes on militant positions. But officials said no definitive decision had been made.
Sunni insurgents allied with the Islamic State of Iraq and Syria, an Al Qaeda breakaway group, overran Mosul and other areas of northern and central Iraq last week in a lightning-quick advance.
Early Wednesday, authorities said, militants attacked the huge refinery complex at Baiji, 125 miles north of Baghdad, the capital. Fighting at the site was said to be continuing. Some storage facilities were reportedly set ablaze.
Iraqi officials denied reports that the refinery had been overrun and said government forces had repelled the attack. But multiple Western news outlets, citing unidentified refinery officials, reported that militants had gained control of 75% of the facility in the predawn assault.
The area near the refinery complex had been the site of clashes in recent days, leading to a halt in production. Many workers have been evacuated from the complex, news agencies reported. But Wednesday's assault appeared to represent a major effort by insurgents to assert control over the refinery.
The refinery would be a key strategic prize for the insurgents, who already have taken over oil fields in eastern Syria. It was unclear whether they would try to operate the facility, destroy it or leave it shuttered.
The Baiji refinery is a major supplier of gasoline and other fuels to the Iraqi domestic market and a key link in the nation's energy supply chain. Its disruption could aggravate the scarcity of electric power and the availability of gasoline and other fuels in Iraq.
After being battered by war, Iraq's oil production has rebounded to become the second-largest among OPEC nations, and it is the backbone of the country's economy.
Severe gasoline shortages were evident Wednesday in much of northern Iraq. Lines of motorists stretched two miles or more at gas stations in Irbil, in the semiautonomous Kurdish region.
In his nationally televised address, Maliki asserted that the military was making gains after discouraging setbacks. Many Iraqi troops abandoned their posts last week in the face of the insurgents' advance, raising fear that the militants could sweep into heavily fortified Baghdad.
"What happened to Iraq was a catastrophe and not every catastrophe is a defeat, for Iraq has regained its national unity," said the prime minister, who has both Iranian and U.S. support. "We have started our counteroffensive, regaining the initiative and striking back."
On Tuesday, the government said its forces had beaten back insurgents' attempt to overrun the city of Baqubah, just 35 miles northeast of the capital.
Baqubah is the capital of Diyala province, whose mixed population includes Shiites, Sunnis and Kurds. The Sunni-led insurgents have so far made most of their gains in regions where Sunni Muslims are the majority. The insurgents are likely to face a greater challenge as they approach Baghdad, home to a huge Shiite Muslim population.
In Baghdad, tens of thousands of Shiite men were reported to be signing up for military service and for pro-government militias in a bid to halt the advance.
Critics have accused Maliki, a Shiite, of governing in a sectarian fashion that has marginalized Iraq's minority groups, including Sunnis, while favoring the Shiite majority.
The Iraqi prime minister has denied having a sectarian agenda and has accused Saudi Arabia, the major Sunni power in the Persian Gulf region, of inciting the violence in Iraq.