GENEVA (Reuters) - The U.N. aid coordinator called on the Saudi-led coalition to open all Yemen s seaports urgently on Tuesday, saying millions of lives were at risk.  The Saudi-led coalition fighting Yemen s Houthi movement said last week it had closed all air, land and seaports in Yemen to stem what it said was the flow of arms to the Houthis from Iran.  We have some 21 million people needing assistance and seven million of those are in famine-like conditions and rely completely on food aid,  U.N. humanitarian coordinator for Yemen Jamie McGoldrick said.   The continued closure by the Saudi-led coalition of critical seaports and airports is aggravating an already dire humanitarian situation. I think it poses a critical threat to the lives of millions who are already struggling to survive.   McGoldrick was speaking to reporters in Geneva by phone from Amman, because he said flights into Sanaa were blocked.  The humanitarian impact of what is happening right now is unimaginable,  he said.  The Saudi-led coalition has said it will keep Hodeidah port closed until a U.N. verification programme is reviewed to ensure no weapons reach the Houthis.  Iran denies arming the Houthis and blames the two-and-a-half-year conflict in Yemen on Riyadh.  McGoldrick said the Saudi plan to supply Yemen through the Saudi port of Jizan in the north and Aden in the south was too complicated, dangerous, slow and expensive, adding an estimated $30 per tonne to every shipment.   We would ask that the coalition opens all the seaports as a matter of urgency and allows humanitarian and other supplies to move, as well as the movement of aid workers,  he said. Humanitarian agencies had been successful in preventing famine and tackling a cholera outbreak that has sickened more than 900,000 people in six months and killed over 2,200.  This import blockage will reverse those gains and leave millions of people in a very precarious situation as we move ahead. The humanitarians are just holding things together, waiting for a peace process which is very much in the distance.  The north of the country, home to 78 percent of the population, had 20 days  stocks of diesel, crucial for pumping water and fighting cholera, and 10 days  stocks of gasoline, with no prospect of resupply soon, he said. Yemen had commercial wheat stocks for three months for the entire population of 28 million and about 120 days of rice. The U.N. children s agency UNICEF had only three weeks of vaccine supplies left in Yemen, and both UNICEF and the World Health Organization had shipments of essential medicines and vaccines blocked in Djibouti, McGoldrick said. Yemen s national airline said on Tuesday a commercial flight had landed at Aden international airport after acquiring security permits. 