A report by a London-based think tank suggests that Iran has supplied Iraq with warplanes to bolster its forces as it fights an offensive by Sunni-led Islamist militants.
The International Institute for Strategic Studies (IISS) says imagery analysis of a video released by the Iraqi authorities suggests the Sukhoi jets delivered on July 1 originated from Iran.
Russia earlier supplied an initial delivery of Su-25 ground attack planes -- a single-seat, twin-engine jet aircraft developed in the Soviet Union in the late 1970s.
According to Joseph Dempsey, an analyst for the IISS's "Military Balance" publication, it is unclear who will be responsible for crewing and maintaining the aircraft.
But the author of the report, dated July 2, added that “it seems increasingly unlikely that Iraq retains the capacity to operate this type of aircraft in any significant number without some level of external support.”
Iraqi government troops are battling militants led by the the Islamic State in Iraq and the Levant (ISIL), which have overrun large swaths of territory in Syria and northern Iraq, and on June 29 declared a "caliphate," or Islamist state, on the area under its control.
Washington has deployed drones and helicopters to Iraq and is also providing Iraq's existing air force with Hellfire missiles.
Analysts say that the likely presence of Iranian aircraft would mean that the United States is operating alongside arch-rival Iran in the conflict.
Iran has several Su-25 warplanes, operated by the Islamic Revolutionary Guards Corps (IRGC).
U.S. State Department spokeswoman Jen Psaki declined to comment on the report but said the United States maintains its "concerns about the type of help Iran may or may not want to supply" to Iraq.
On the ground, Iraqi forces were struggling to break a stalemate with ISIL-led militants.
Thousands of troops, backed by tanks, artillery, and aerial cover, have made limited progress in retaking the northern city of Tikrit, which fell on June 11.
Ahmed Abdullah Juburi, the governor of Salaheddin Province, of which Tikrit is the capital, said the troops' advance into the city has been slowed down by booby-trapped buildings and "lots of roadside bombs and car bombs."
Meanwhile, Iraqi Prime Minister Nuri al-Maliki has called ISIL's declaration of a caliphate a threat to the entire region.
In his weekly address on July 2, Maliki said the announcement by ISIL that it has unilaterally established a caliphate "is a message to all the states in the region that you are inside the red circle now."
Maliki said "no one in Iraq or any neighboring country will be safe from these plans."
Maliki also said that he rejected an assertion by the country's autonomous Kurdish region that it will maintain control of disputed territory it has occupied amid the ISIL drive.
"No one has the right to exploit the events that took place to impose a fait accompli, as happened in some of the actions of the Kurdistan region," Maliki said.
Kurdish forces say they assumed control of the disputed territory in and around Kirkuk -- a major oil hub -- to prevent it from being taken over by the Sunni insurgents as Iraqi government troops retreated.
In his speech, Maliki also offered an amnesty to some of the Sunni tribes who have supported the militant offensive.
Maliki's speech came a day after the first session of Iraq's new parliament on July 1 ended in disarray without settling on a new speaker.