NAIROBI, Aug 20 (Reuters) - For the first time in nine months of war in Ethiopia"s Tigray region, aid workers will run out of food this week to deliver to millions of people who are going hungry, the head of the U.S. government"s humanitarian agency said, blaming the government for restricting access. "USAID and its partners as well as other humanitarian organizations have depleted their stores of food items warehoused in Tigray," Samantha Power, head of the U.S. Agency for International Development (USAID), said in a statement late on Thursday. "People in Tigray are starving with up to 900,000 in famine conditions and more than five million in desperate need of humanitarian assistance," Power said. "This shortage is not because food is unavailable, but because the Ethiopian Government is obstructing humanitarian aid and personnel, including land convoys and air access." War broke out in November between Ethiopian troops and the Tigray People"s Liberation Front (TPLF), which controls the region. The conflict has killed thousands and sparked a humanitarian crisis in one of the world"s poorest regions. Billene Seyoum, spokesperson for Prime Minister Abiy Ahmed, did not respond to a request for comment. At a news conference on Friday, she did not refer to Power"s statement but dismissed allegations that the Ethiopian government is "purposely blocking humanitarian assistance", saying the government is concerned about security. "It is important to really address this continuing rhetoric because that is not the case," Billene said. "Security is first and foremost a priority that cannot be compromised, it is a volatile area so in that regards there is going to be continuous checks and processes." On Thursday, U.N. Secretary-General Antonio Guterres called for an immediate ceasefire and unrestricted aid access in Tigray. The U.N. warned last month that more than 100,000 children in Tigray could die of hunger. read more Power"s statement said that 100 trucks carrying food and life-saving supplies need to be arriving each day in Tigray to meet the humanitarian needs there. As of a few days ago, only about 320 trucks had arrived, less than 7% of what is required, it said. "Across all sectors, available aid supplies in Tigray are extremely depleted, and humanitarian operations may be forced to cease if more ... aid convoys do not arrive imminently," Saviano Abreu, spokesperson for U.N. Office for the Coordination of Humanitarian Affairs (OCHA) told Reuters on Friday. The Ethiopian government declared a unilateral ceasefire in June after Tigrayan forces recaptured the regional capital Mekelle and retook most of the region. The Tigrayan forces dismissed this as a "joke" and issued preconditions for truce talks. Reporting by Maggie Fick Additional reporting by Ayenat Mersie and Giulia Paravicini Editing by John Stonestreet, Frances Kerry and Catherine Evans Our Standards: The Thomson Reuters Trust Principles.