A superyacht owned by the Russian billionaire Alexei Mordashov was able to transit the blockaded strait of Hormuz after undergoing maintenance in Dubai because neither Iran nor the US objected, a source close to Mordashov said on Tuesday.

It has been unclear how the multi-deck pleasure vessel, worth more than $500m (£370m), gained permission to sail on Saturday through the commercially important waterway at the heart of the US-Iran conflict, where traffic has been severely restricted since February.

Alexei Mordashov. Photograph: Kommersant photo agency/Rex/Shutterstock

Sailing under a Russian flag, the yacht, called Nord, crossed the strait on an approved route in compliance with international maritime law, the source said.

“Iran did not interfere with the movement of the yacht, as it is a civilian vessel of a friendly country conducting a peaceful transit. The American side also raised no questions regarding the yacht’s movement, as it did not call at Iranian ports and has no connection to Iran,” the source said.

Just a few, mainly merchant vessels, have been able to pass through the crucial waterway at the entrance to the Gulf as Washington and Tehran maintain an uneasy ceasefire. This is a fraction of the average 125 to 140 daily passages before the war began on 28 February. In response, the US has blockaded Iranian ports.

Russia is a longstanding ally of Iran. The Iranian foreign minister, Abbas Araghchi, travelled to St Petersburg on Monday for a meeting with the Russian president, Vladimir Putin, after discussions with peace mediators in Pakistan and Oman at the weekend.

After crossing the strait, Nord has been located near the coast of Oman since Sunday, according to the data provider LSEG.