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The supply of weapons to Iraqi Kurdistan comes as Kurdish fighters struggle to stem advances by militants from the Islamic State, an al Qaeda offshoot.
The sources said the weapons were supplied by the Central Intelligence Agency but that the Department of Defense may soon start arming the Kurdish fighters, who regained control of two strategic towns in northern Iraq on Sunday with help from US airstrikes.
The officials declined to specify when the supply program began or what sort of arms it included.
Weapons have also been shipped in three deliveries from the Iraqi government in Baghdad to Arbil, consisting mostly of AK-47 assault rifles and ammunition, the US officials said. Reuters was first to report on Friday that the Iraqi government had sent a first, unprecedented shipment of ammunition to Arbil.
The United States has long insisted that all sales of US weapons must go through Iraq's central government, despite Kurdish complaints that Baghdad had deprived them of promised military equipment and financial support.
Critics accuse US President Barack Obama of being reluctant or too slow to intervene in thorny foreign policy issues which have piled up under his watch, including the dramatic rise of the Islamic State, which has seized control of large swathes of land in the north and west of OPEC member Iraq.
A senior US defense official acknowledged that the US was providing arms and ammunition needed by the Kurds but said it was not coming from the Department of Defense. Officials said the Pentagon was having discussions about how to increase its military support to the Kurds and could soon approve a decision to directly supply weaponry.
FIRST SINCE 2011
Just last week Washington launched its first military action in Iraq since pulling its troops out in 2011. US warplanes bombed Sunni insurgents from the Islamic State, who have marched through northern and western Iraq since June.
Washington says it is taking limited action to protect the Kurdish autonomous region and prevent what Obama called a potential "genocide" of religious minorities targeted by the militants.
The militants made new gains against Kurdish forces despite three days of US airstrikes, while Baghdad, long braced for the Sunni fighters to attack, was now tensing for possible clashes between forces loyal to Prime Minister Nuri al-Maliki and those of his rivals after Iraq's president named a new prime minister on Monday.
Obama says a more inclusive government in Baghdad is a precondition for more aggressive US military support against the Islamic State. He has rejected calls in some quarters for a return of US ground troops, apart from several hundred military advisers sent in June.
The Islamic State, which sees Shi'ites as heretics who deserve to be killed, has ruthlessly moved through one town after another, using tanks and heavy weapons it seized from soldiers who have fled in their thousands.
On Monday, police said the militants had seized the town of Jalawla, 115 km (70 miles) northeast of Baghdad, after driving out the Kurds' Peshmerga forces.
Washington and its European allies are considering requests for more direct military aid from the Kurds, who have themselves differed with Maliki over the division of oil resources and who took advantage of the Islamists' advance to expand their territory.