WASHINGTON (Reuters) - The United States has warned Saudi Arabia that concern in Congress over the humanitarian situation in Yemen could constrain U.S. assistance, as it pushed Riyadh to allow greater access for humanitarian aid, a U.S. official said on Friday.  The Saudi-led military coalition fighting the Iran-aligned armed Houthi movement in Yemen s civil war started a blockade of ports a month ago after Saudi Arabia intercepted a missile fired toward its capital Riyadh from Yemen. Although the blockade later eased, Yemen s situation has  remained dire. About 8 million people are on the brink of famine with outbreaks of cholera and diphtheria. That has led the White House to take the rare step of issuing two written statements in a week on Yemen, including one on Friday calling on the Saudi-led coalition to help facilitate the free flow of humanitarian aid and critical goods, like fuel.  I think there has just been mounting concern over the continued humanitarian conditions in Yemen, and while we have seen progress, we haven t seen enough,  said a senior Trump administration official, speaking on condition of anonymity.  We want to see more in the coming weeks.  The White House also called on the Houthis to allow food, medicine and fuel to be distributed and accused them of political repression and brutality.  The Yemen war s heavy toll on civilians has long been a sore point with members of Congress, triggering threats to block U.S. assistance to the Saudi-led coalition. That includes U.S. refueling of coalition jets and the provision of limited U.S. intelligence support.    Senator Chris Murphy, a Democrat from Connecticut and longtime critic of U.S. support for the Yemen campaign, cheered  Trump s push for humanitarian aid this week. But in the same breath, he warned about U.S. assistance to the Saudis.  The Trump administration must continue to make clear to Saudi Arabia that the U.S. will not support a campaign that intentionally starves civilians into submission,  Murphy said. Trump administration officials have underscored those concerns in Congress to Riyadh.  We wanted to be very clear with Saudi officials that the political environment here could constrain us if steps aren t taken to ease humanitarian conditions in Yemen,  the official said. Publicly, Trump, his top aides and senior Saudi officials have hailed what they say is a major improvement in U.S.-Saudi ties compared with relations under former President Barack Obama, who upset the Saudis by sealing a nuclear deal with their arch-foe Iran. Even as ties improve, however, U.S. diplomats and intelligence analysts privately have expressed anxiety over some of the more hawkish actions by Saudi Arabia s crown prince, especially toward Yemen and Lebanon, as Saudi Arabia seeks to contain Iranian influence. In turn, Saudi Arabia has been unusually public about its concerns over President Donald Trump s move to recognize Jerusalem as the capital of Israel. The Trump administration shares Saudi Arabia s concerns over Iran and emphasized that point on Friday. In its statement, the White House squarely blamed Iran s Islamic Revolutionary Guard Corps and its partners for  arming, advising, and enabling the Houthis  violent actions.   We condemn the Houthis  brutal repression of political opponents in Sanaa,  Friday s statement by White House spokeswoman Sarah Sanders said. That includes the Houthis  killing last weekend of Ali Abdullah Saleh, Yemen s former president, to punish him for  switching sides in Yemen s three-year civil war. The killing was a setback for Riyadh, which had hoped the backing of Saleh   and his loyalist army units in northern Yemen - would help close a war that has killed 10,000 people and caused one of the world s most acute humanitarian crises. The White House said political negotiations were necessary to end violence in the country,  free of the malign influence of Iranian-backed militias.   The Iranian-backed Houthi militias must allow food, medicine, and fuel to be distributed throughout the areas they control, rather than diverted to sustain their military campaign against the Yemeni people,  Sanders added. 