Militants appeared to have captured five Iraqi soldiers near Fallujah, according to witnesses and online videos, while anti-government fighters took control of more territory in Anbar province on Sunday.
Meanwhile attacks elsewhere in Iraq killed 13 people, pushing to more than 850 the number of people killed this month.
Authorities have been grappling for weeks with a deadly standoff in Anbar province, west of Baghdad.
Foreign leaders have urged the Shiite-led government to seek political reconciliation with disaffected minority Sunnis in order to undercut support for militants.
But with elections looming in April, Iraqi Prime Minister Nuri al-Maliki has taken a hard line.
On Sunday, witnesses said between five and 22 soldiers, were captured by anti-government fighters who also seized several military vehicles.
The alleged operations were shown in videos posted on the YouTube, but their authenticity could not be immediately verified
Witnesses said that anti-government fighters attacked a small military outpost on the outskirts of Fallujah in the morning, forcing some soldiers to retreat while others surrendered.
One video shows five men dressed in Iraqi army uniforms sitting in the back of a pick-up truck as onlookers crowd around them.
The men hoist a black flag akin to those often flown by jihadists and shout slogans in support of the Al-Qaeda-linked Islamic State of Iraq and the Levant (ISIL).
Another video shows militants driving two army Humvees as Fallujah residents look on.
Witnesses said as many as six Humvees were seized, and that three were set ablaze.
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Elsewhere in Anbar, militants overran police stations in Albubali and Albu Obeid, rural areas between Fallujah and nearby Ramadi, after elite security forces withdrew from the area, police officers aid.
The officers, who spoke on condition of anonymity, said policemen fled the area with their families after they were surrounded by anti-government fighters, and added that weapons and equipment were also left behind.
The latest unrest marks another series of setbacks for Iraqi forces who have struggled to wrest back control of Fallujah and parts of Anbar provincial capital Ramadi that have been out of government control for weeks.
Security forces have also been locked in deadly battles in Ramadi, where militants hold several neighbourhoods, and have carried out operations in rural areas between the two cities.
ISIL has been involved in the fighting, and witnesses and tribal leaders in Fallujah say the group has tightened its grip on the city in recent days. Other militant groups have also been involved in the battles.
Also in Fallujah, a mother and her three children were killed when a blast struck their home in the south of the city in the early hours of Sunday, Dr. Ahmed Shami said.
It was unclear if heavy artillery or smaller rockets were responsible for the blast.
It is the first time militants have exercised such open control in Iraqi cities since the peak of the violence that followed the 2003 US-led invasion.
The protracted standoff in Anbar has forced more than 140,000 people to flee, the UN refugee agency said, describing this as the worst displacement in Iraq since the 2006-08 sectarian conflict.
Elsewhere in Iraq Sunda, nine people were killed in gun and bomb attacks, officials said.
Three apparently coordinated car bombs struck Kirkuk, a disputed ethnically-mixed city in north Iraq, killing four people. Attacks in and around Baghdad left four more dead, including a former army general.
In the main northern city of Mosul, a tribal leader was gunned down in his car.
The latest bloodshed pushed the overall death toll for the month above 850 -- more than three times the toll for January 2013, according to an AFP tally.