Iran has assured Kuwait it will not try to close the vital Strait of Hormuz shipping route, Kuwait's ruler said in remarks carried by state-run news agency Kuna on Tuesday.

Sheikh Sabah Al-Ahmad Al-Sabah said Kuwait nevertheless had been working for "a long time" on building up oil stocks outside the Gulf to ensure steady supplies to customers.

After threats by Iran that it could shut the most important oil transit channel in the world, if Western governments stop it from selling crude, Kuwait's emir and other Gulf leaders have sought assurances that Tehran will not follow through with the threats.

"(We) have contacted officials in Iran to ensure that no action is taken to close the Strait of Hormuz," according an English version of his remarks to Japanese press distributed by Kuwait state news agency Kuna.

"We have received assurances from Iran that it will not take this step," the emir said during a visit to Japan, one of the Gulf oil exporter's biggest customers.     

"For a long time, Kuwait has been working on providing a stockpile of oil through its global companies outside the Gulf region to ensure constant supply," he said.

Several Iranian officials have said Iran should block the waterway in response to sanctions targeting its nuclear programme. Western governments suspect Iran is trying to make atomic weapons. Tehran denies this.

Opec member Kuwait, which is producing around 3 million barrels a day, ships all its oil exports through Hormuz. - Reuters