Saudi Arabia's crown prince said Iran's supply of rockets to militias in Yemen is an act of "direct military aggression" that could be an act of war, state media reported on Tuesday, remarks reflecting sharply heightened strains between Riyadh and Tehran.Prince Mohammed bin Salman's comments were published after Saudi air defense forces intercepted a ballistic missile they said was fired towards Riyadh on Saturday by the Iran-allied Houthi militia which controls large parts of neighboring Yemen.Saudi-led forces, which back the internationally-recognized government, have been targeting the Houthis in a war which has killed more than 10,000 people and triggered a humanitarian disaster in one of the region's poorest countries. The supply of rockets to the Iran-allied rebel Houthi movement could "constitute an act of war against the Kingdom," state news agency SPA on Tuesday quoted Prince as saying in a call with British Foreign Minister Boris Johnson.Iran has denied it was behind the missile launch, rejecting the Saudi and US statements condemning Tehran as "destructive and provocative" and "slanders".In reaction to the missile, the Saudi-led military coalition said on Monday it would close all air, land and sea ports to the Arabian Peninsula country.Human Rights Watch said on Tuesday the missile launch was "most likely a war crime" but urged Saudi Arabia against restricting aid access to Yemen, where the United Nations estimates some seven million people are on the brink of famine and nearly 900,000 infected with cholera. "This unlawful attack is no justification for Saudi Arabia to exacerbate Yemen’s humanitarian catastrophe by further restricting aid and access to the country," it said.The coalition said aid workers and humanitarian supplies would continue to be able to access and exit Yemen despite the temporary closure of ports but the UN said it was not given approval for two scheduled humanitarian flights on Monday.The United Nations and international aid organizations have repeatedly criticized the coalition for blocking aid access, especially to northern Yemen, which is held by the Houthis.In an interview with CNN television on Monday, Saudi Foreign Minister Adel al-Jubeir accused the armed Lebanese Hezbollah group of firing the missile at Riyadh from Houthi-held territory."With regards to the missile ... that was launched on Saudi territory, it was an Iranian missile launched by Hezbollah from territory occupied by the Houthis in Yemen."He said the missile was similar to one launched in July at Yanbu in Saudi Arabia and was manufactured in Iran, disassembled and smuggled into Yemen, then reassembled by the operatives of the Iranian Revolutionary Guards and Hezbollah, "then it was launched into Saudi Arabia."The Saudi-led coalition has been targeting the Houthis since they seized parts of Yemen in 2015, including the capital Sanaa, forcing President Abd-Rabbu Mansour Hadi to flee and seek help from neighboring Saudi Arabia.