Two container ships seized by Iran near the Strait of Hormuz with about 40 crew aboard have been taken toward the port of Bandar Abbas, sources said on Thursday, Tehran having vowed to retaliate after U.S. forces ‌seized an Iranian vessel three days earlier.

Iran's Islamic Revolutionary Guard Corps seized the ships on Wednesday, ​one operated by MSC, the world's biggest container shipping group, and according to three sources, the other chartered by the firm.

"Some 20 Iranians armed to the teeth stormed the ship. Sailors ⁠are under Iranians' control, their movements on the ship are limited, but the Iranians are treating them ​well," a relative of one of the seafarers involved told Reuters.

"The ship is anchored nine nautical miles from ⁠the Iranian coast. Negotiations between MSC and Iran are ongoing, our sailors are fine," Montenegro's minister of maritime affairs, Filip Radulovic, told state broadcaster state RTCG.

Four sailors on the MSC Francesca including its captain are from Montenegro, he said. Two Croatians are also aboard, Croatia's foreign ministry ‌confirmed.

Full details on the crew of the Panama-flagged ship have not been made available, but large ​container vessels normally require ‌a minimum of about 20. MSC has declined to comment.

The Liberia-flagged Epaminondas has a crew of 21 members made up of Ukrainians and Filipinos, according to ‌the Greek coast guard. It had been bound for India.

Both crews were reported safe, but authorities in their home countries said they were seeking information about the well-being of the seafarers and working on their release.

There ⁠has been no information released about what, if ‌any, cargo the ships were carrying.

Both ships ⁠had their tracking transponder systems switched off but maritime security sources said shipping data suggested they were near Bandar Abbas.

On April 19, ⁠U.S. ⁠forces fired upon and seized the Iran-flagged Touska cargo ship.

"The armed forces of the Islamic Republic of Iran will soon respond and retaliate against this armed ‌piracy by the U.S. military," a military spokesperson said in response.

Iran's foreign ministry demanded the immediate release of the vessel, its crew and their families. No details about its crew have been released.

Oil prices have jumped on dimmed prospects for ‌a quick reopening of ​the strait, a route which usually ‌handles 20% of the world's daily oil and liquefied natural gas supply.

Benchmark Brent crude oil futures were up 2% at $104 a barrel on Thursday versus $72 before the war began on Feb 28.

U.S. ​Central Command said on Wednesday that since the start of its blockade against ships entering or exiting Iranian ports, U.S. forces had directed 31 vessels to turn around or return to port.