The US international development agency has blamed the Ethiopian government for a shortage of humanitarian aid in the country's conflict-torn Tigray region. USAID accused the government of "obstructing" access to Tigray, as it warned that food aid was set to run out this week for the first time. Hundreds of thousands of people are at risk of famine amid the conflict between government and rebel forces. Ethiopia has denied "purposely" blocking aid. Prime Minister Abiy Ahmed's spokeswoman told reporters that the government was allowing aid convoys to enter Tigray, but that security was a "priority that cannot be compromised". "It is a volatile area so... there [are] going to be continuous checks and processes," said Billene Seyoum. In her statement, USAID chief Samantha Power described the flow of humanitarian assistance into the northern region as "woefully insufficient". She said food warehouses were "virtually empty" and that aid workers would soon have nothing to distribute. "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," she said. USAID called on the Ethiopian government to "immediately allow humanitarian assistance". It noted that aid trucks have been unable to leave the town of Semera in the neighbouring Afar region - currently the only accessible land route into Tigray. The United Nations has separately told the BBC more than 100 aid trucks are stuck on this route. About 100 trucks of aid are needed in Tigray each day, yet just 320 have managed to reach the region since the end of June, a UN spokesman told the BBC. "At least two important aid organizations have already run out of food," said Saviano Abreu from the UN humanitarian agency, Ocha. "Without urgent and unimpeded food assistance, there will be an imminent threat to the lives of over 400,000 people in Tigray already in famine-like conditions and over 1.8 million people now in emergency levels of hunger could slide into starvation," he said.