US President Donald Trump openly mused about seizing Iran's Kharg Island oil terminal in the Persian Gulf as America and Israel kept up their attacks Monday on the Islamic Republic, even as there were signs of progress in nascent ceasefire talks.

Tehran, meanwhile, struck a key water and electrical plant in hard-hit Kuwait, part of its ongoing campaign targeting the Gulf Arab states.

As a diplomatic effort being facilitated by Pakistan toward ending the war moved ahead, Trump said Iran had agreed to allow 20 oil tankers through the Strait of Hormuz starting Monday as "a sign of respect."

At the same time, with 2,500 US Marines now in the region and a similar sized contingent on its way, he raised the idea of taking Iran's Kharg Island.

"Maybe we take Kharg Island, maybe we don't," Trump told the Financial Times in an interview published early Monday. "We have a lot of options."

The US President told reporters aboard Air Force One late Sunday that the US was negotiating "directly and indirectly" with Iran, though Tehran has insisted that it has not been in any talks with Washington.

"We're doing extremely well in that negotiation but you never know with Iran because we negotiate with them and then we always have to blow them up," Trump said.

Earlier, Iran's parliament speaker, Mohammad Bagher Qalibaf, dismissed the talks in Pakistan as a cover after more US troops get to the area. He said Iranian forces were "waiting for the arrival of American troops on the ground to set them on fire and punish their regional partners forever," according to state media.

Trump suggested it could mean a longer-term commitment if the US decided to try and take Kharg Island, saying "it would mean we had to be there for a while."

"I don't think they have any defence," he told FT. "We could take it very easily."

To get an amphibious invasion force to Kharg would mean transiting the Strait of Hormuz and most of the Persian Gulf.

Experts say that holding the island would also be a challenge, because in addition to missiles and drones it would be well within artillery range from the Iranian mainland.

Several hundred special operations personnel have arrived in the region, The New York Times reported on Sunday, citing two military officials. That comes on top of thousands of US Marines that came on Friday aboard an amphibious assault ship, the first of two contingents, the US military has said.

Reuters has reported that the Pentagon has been considering military options that could include ground forces, although Trump has not approved any of those plans, according to multiple news outlets.

Kharg island handles 90 per cent of Iran's ​oil exports and seizing it would give the United States the ability to severely disrupt Iran's energy trade, placing ​enormous pressure on Tehran's economy.

Siren in Israel’s nuclear research centre

Sirens sounded at dawn near Israel's main nuclear research centre, a part of the country that has been targeted repeatedly in recent days.

The Israeli military said on Monday that Iran launched multiple waves of missiles at Israel, and an attack had also been launched from Yemen for only the second time since the US-Israeli war began.

It said two drones from Yemen were intercepted early Monday but gave no further details. Yemen's Iran-backed Houthi militia entered the war on Saturday, firing missiles at Israel, escalating a conflict that has engulfed the Middle East.

Israel's military said its air force was carrying out strikes on Tehran on Monday, targeting what it described as military infrastructure.

Iran kept up the pressure on its Gulf Arab neighbours, as Saudi Arabia intercepted five missiles targeting its oil-rich Eastern Province, Bahrain sounded a missile alert, and a fireball erupted over Dubai as an incoming missile was taken out by defences.

In Kuwait, an Iranian attack hit a power and desalination plant, killing one worker and injuring 10 soldiers, the state-run KUNA news agency reported.

Desalination plants remain crucial to water supplies in the Gulf Arab states, and an Iranian attack previously damaged a desalination plant in Bahrain during the war. The facilities are typically paired with power plants, because of the large amount of energy required to remove salt from the water to make it drinkable.

In Lebanon, which Israel has invaded by ground, an Indonesian peacekeeper was killed and three others wounded when a projectile exploded near a village in the south.

Oil prices rise again

In early trading, the spot price of Brent crude oil, the international standard, was around $115, up nearly 60 per cent from when the US and Israel started the war with attacks on Iran on February 28.

Pakistan announced Sunday that it would soon host talks between the US and Iran, though there was no immediate word from Washington or Tehran, and it was unclear whether discussions on the monthlong war would be direct or indirect.

Pakistan's Foreign Minister Ishaq Dar said the talks would be held "in the coming days."