The shaky ceasefire in the Iran war was tested again on Sunday when a cargo ship caught fire after being hit by an unknown projectile off Qatar's coast, and Kuwait's military reported an attack by drones, without specifying where they came from.
The attacks were the latest threats to a month-old ceasefire, which the Trump administration says remains in effect.
It has faced difficulties, with Iran restricting traffic through the Strait of Hormuz, a strategic waterway key to the global flow of oil, and the US imposing a blockade of Iranian ports.
On Friday, the US struck two Iranian oil tankers after it said that the vessels were trying to breach its blockade of Iran's ports.
Iran's Revolutionary Guard navy on Sunday repeated its warning that any attack on Iranian oil tankers or commercial vessels would be met with a "heavy assault" on one of the US bases in the region and enemy ships.
US President Donald Trump has reiterated threats to resume full-scale bombing, if Iran doesn't accept an agreement to reopen the strait and roll back its nuclear program.
Japan