Iran is stepping up weapons transfers to Houthi rebels in Yemen via smuggling routes through Oman, despite the current ceasefire in the country's civil war, the Reuters news agency reported on Thursday citing diplomatic sources.
A senior US official told Reuters that Washington had conveyed its concerns to Oman about the suspected flow of weapons into neighbouring Yemen.
Another western diplomat said there had been a "recent increased frequency of weapons shipments supplied by Iran, which are reaching the Houthis via the Omani border".
"What they're bringing in via Oman are anti-ship missiles, explosives..., money and personnel," another US official said.
Oman has denied its territory is being used as a smuggling route by the Houthis with Omani Foreign Minister Yousef bin Alwi last week saying there was "no truth" in the claim.
Yemeni and senior regional officials told Reuters the Omanis were turning a blind eye and failing to aggressively crack down on the flow.
Both Saudi Arabia and the US have accused Iran of supplying weapons to the Houthis, but Tehran has denied doing so.
Reuters however quoted a senior Iranian diplomat who said there had been a "sharp surge in Iran's help to the Houthis in Yemen" since May, referring to weapons, training and money.
Saudi Arabia is leading a coalition of Arab states that has been battling the Houthis and forces loyal to former Yemeni president Ali Abdullah Saleh since March 2015 in support of Yemen's exiled President Abd Rabbuh Mansour Hadi.
The US has recently launched retaliatory strikes against Houthi targets following a missle attack targeting a US warship in the Red Sea.
Sporadic fighting was reported on Thursday hours after a United Nations-backed three-day ceasefire plan came into effect.
Both sides have said they will respect the ceasefire providing the other side also does so.
It is the sixth attempt to end the bloodshed since Saudi Arabia and its allies entered the conflict.
The last ceasefire attempt began in April and later collapsed alongside UN-brokered peace talks in Kuwait.
Almost 6,900 people have been killed -- more than half of them civilians -- while another three million are displaced and millions more need food aid.
EU foreign policy chief Federica Mogherini said late on Wednesday that the truce should be a first step towards resuming peace talks.
"The ceasefire must be respected by all sides and its duration extended so as to create the necessary conditions for such negotiations," she said.