US Secretary of State John Kerry has warned Iran over its alleged support for Houthi rebels in Yemen.
He said the US would support any state in the Middle East that felt threatened by Iran, and would not “stand by” if Iran destabilised the region.
The US is backing a Saudi-led coalition seeking to drive back the rebels and restore President Abdrabbuh Mansour Hadi, who fled the country last month.
Iran has denied accusations it is providing military aid to the Houthis.
However, it despatched two navy vessels to the Gulf of Aden, off the southern coast of Yemen, on Wednesday.
Navy commander Rear Adm Habibollah Sayyari told state media the move was made with the aim of “safeguarding naval routes for vessels in the region”.
Meanwhile, aid supplies have begun reaching Yemen after a series of delays, as the humanitarian situation deteriorates amid continued fighting.
Speaking to PBS Newshour on Wednesday, Mr Kerry said it was clear Iran was aiding the Houthis: “There are obviously supplies that have been coming from Iran. There are a number of flights every single week that have been flying in.”
“Iran needs to recognise that the US is not going to stand by while the region is destabilised or while people engage in overt warfare across lines, international boundaries and other countries,” he added.
Fighting has intensified in the city of Aden between the Houthis and militiamen loyal to President Hadi.
Meanwhile, warplanes from the Saudi-led coalition have continued to bomb rebel targets across the country.
On Thursday, Iranian President Hassan Rouhani called for a halt to the air campaign and said countries in the region should work together to resolve the crisis.
“A great nation like Yemen will not submit to bombing. Come, let us all think about ending war. Let us think about a ceasefire,” he said in a speech. “Let us prepare to bring Yemenis to the negotiating table to make decisions about their future.”
Mr Rouhani warned Saudi Arabia and its allies that their intervention was a mistake.
“Why have you started killing people?” he asked. “You do not know how this will end. This path is wrong.”
The coalition has so far failed to stop an assault on the southern port city of Aden by the Houthis and allied military units loyal to former President Ali Abdullah Saleh.
The president took refuge there in February after fleeing the capital, Sanaa, where he had been held under house arrest by the rebels. When the Houthis seized an airbase outside Aden, he left for Saudi Arabia.
On Wednesday, medics said at least 22 people were killed as the rebels fought Mr Hadi’s forces for control of Aden’s central Crater district and coalition warplanes targeted rebel positions in its northern suburbs.
Conditions for civilians in the city have been described as catastrophic by aid groups.
Two ships carrying humanitarian aid were able to reach Aden on Wednesday for the first time since the coalition air campaign began on 25 March.
One vessel brought 1.7 tonnes of medical aid for Medecins Sans Frontieres (MSF), and the other medical supplies and a surgical team from the International Committee of the Red Cross (ICRC).
Further aid from the ICRC is expected to be delivered by plane to Sanaa shortly.
The World Health Organisation says at least 643 people have been killed and 2,226 wounded in Yemen since 19 March. Another 10,000 people have been driven from their homes.
Houthis – The Zaidi Shia Muslim rebels from the north overran Sanaa last year and then expanded their control. They want to replace Mr Hadi, whose government they say is corrupt. The US alleges Iran is providing military assistance to the rebels.
Ali Abdullah Saleh – Military units loyal to the former president – forced to hand over power in 2011 after mass protests – are fighting alongside the Houthis.
Abdrabbuh Mansour Hadi – The president fled abroad in March as the rebels advanced on Aden, where he had taken refuge in February. Loyal soldiers, Sunni Muslim tribesmen and Southern separatists have formed militia to fight the rebels.
Saudi-led coalition – A US-backed coalition of nine, mostly Sunni Arab states says it is seeking to “defend the legitimate government” of Mr Hadi.
Al-Qaeda in the Arabian Peninsula – AQAP opposes both the Houthis and President Hadi. A rival affiliate of Islamic State has also recently emerged.