Iran slammed a US blockade around its ports on Monday as a "grave violation" of its sovereignty, as Washington and Tehran's belligerent rhetoric rattled a fragile truce.

"The imposition of a maritime blockade constitutes a grave violation of the sovereignty and territorial integrity of the Islamic Republic of Iran," Iran's ambassador to the United Nations Amir Saeid Iravani wrote to UN Secretary General Antonio Guterres, in a letter seen by AFP.

The "unlawful" blockade also "constitutes a serious violation of the fundamental principles of the international law of the sea," Iravani added.

US President Donald Trump ordered a blockade of ships entering or leaving Iranian ports and coastal areas in the Gulf Sunday, following the failure of peace talks over the weekend, warning that any Iranian attack boats that challenged the blockade would be destroyed.

The blockade came into force at 1400 GMT Monday, despite the United States and Iran agreeing to a two-week ceasefire days earlier, with mediator Pakistan and Gulf state Qatar continuing calls for it to be respected as diplomatic efforts continue.

Guterres on Monday called on "all parties" to respect freedom of navigation in the Strait of Hormuz, after Iran and subsequently the United States blocked passage through the critical waterway.

The strait, through which a fifth of global oil and gas flows, has become a key flashpoint as the US and Iran attempt to gain leverage over each other during ceasefire negotiations.

Iravani said the "unlawful" blockade "poses a grave threat to international peace and security and obviously exacerbates the risk of escalation in an already highly volatile region."

In a second letter, the UN envoy called on Middle Eastern countries hosting US military bases to "cease their internationally wrongful acts."

Bahrain, Saudi Arabia, Qatar, the United Arab Emirates and Jordan should pay "compensation for all material and moral damage sustained," he said.