Tehran, Iran - Iran's Revolutionary Guards on Thursday threatened to strike American bases across the Middle East if the US military continues to escort ships through the Strait of Hormuz.

"The US military must know that if this trend continues, the conflict will extend to its bases in the region," the Guards, the ideological arm of Iran's military, said in a statement on their Sepah News website.

"This is a serious warning," they added.

Iran claims control over Hormuz, a strategic shipping route through which one-fifth of the world's oil supplies passed before war broke out in February. It regularly attacks ships that attempt to navigate the strait without its authorization.

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But the US military has been escorting vessels through the waterway in increasing numbers, with Iran responding by stepping up attacks.

Four ships were hit in the area overnight as they attempted to pass through the wider Gulf, Iranian forces said on Thursday.

Washington has also been maintaining a naval blockade on Iranian ports in an attempt to squeeze the country's economy, while negotiations to end the conflict are deadlocked.

Hormuz, bordered to the north by Iran and to the south by Oman, is only about 19 miles wide at its narrowest point and is a critical route for ships carrying oil and gas from energy-rich Gulf countries.