By Trevor Hunnicutt, Ariba Shahid and Muhammad Al Gebaly

WASHINGTON/ISLAMABAD, April 18 (Reuters) - Iran said it was tightening control over the Strait of Hormuz on Saturday, warning mariners the vital energy route was again closed, but President Donald Trump said Tehran could not blackmail the United States by shutting the waterway.

Tehran said it was responding to a continued U.S. blockade of Iranian ports, calling it a violation of their ceasefire, while Supreme Leader Mojtaba Khamenei said Iran's navy was ready to inflict "new bitter defeats" on its enemies.

Shipping sources said at least two vessels reported coming under fire and being hit while trying to transit the waterway. India later said the Iranian ambassador in New Delhi had been summoned and that it had expressed deep concern to him that two Indian-flagged ships had come under fire in the strait.

State media in Iran quoted the Supreme National Security Council as saying Iranian control over the strait included demanding the payment of costs related to security, safety and environmental protection services.

State television also quoted the Supreme National Security Council as saying the U.S. had put forward new proposals after talks mediated by Pakistan in recent days. Tehran was considering them but had not yet responded, it said.

There was no immediate sign of direct U.S.-Iran talks taking place at the weekend, despite Trump saying on Friday that negotiations would take place.

UNCERTAINTY AROUND IRAN CONFLICT

Tehran’s renewed tough messaging caused fresh uncertainty around the Iran conflict, raising the risk that oil and gas shipments through the strait could remain disrupted just as Washington weighs whether to extend the fragile ceasefire.

Trump said the U.S. was having "very good conversations" with Iran but that Tehran wanted to close the strait again. Iran could not blackmail the U.S., he said.

Maritime security and shipping sources said some merchant vessels had received radio messages from Iran’s navy saying no ships were allowed through the waterway, reversing Friday's signs that traffic might resume.

Maritime trackers had earlier shown a convoy of eight tankers transiting the narrow passage in the first major movement of ships since the U.S.-Israeli war on Iran began seven weeks ago.

Hours earlier, Trump had cited "some pretty good news" about Iran, declining to elaborate. But he also said fighting might resume without a peace deal by Wednesday, when the two-week ceasefire expires.

Iran had announced its temporary reopening of the Strait of Hormuz following a separate U.S.-brokered 10-day ceasefire agreement on Thursday between Israel and Lebanon. Israel invaded parts of southern Lebanon after the Iran-allied Hezbollah militant group joined the fighting in early March.

But on Saturday Iran's armed forces command said transit through the strait had reverted to a state of strict Iranian military control, citing what it described as repeated U.S. violations and acts of “piracy” under the guise of a blockade.

The spokesperson said Iran had earlier agreed, “in good faith,” to the managed passage of a limited number of oil tankers and commercial vessels following negotiations, but said continued U.S. actions had forced Tehran to restore tighter controls on shipping through the strategic chokepoint.

U.S. Central Command said in a statement that American forces were enforcing a maritime blockade of Iran, but did not comment on the latest Iranian actions.

NO DATE FOR DIRECT TALKS

The war with Iran began on February 28 with a U.S.-Israeli attack on the Islamic Republic. It has killed thousands, spread to Israeli attacks in Lebanon and sent oil prices surging because of the de facto closure of the strait.

Despite the initial movement of ships, Iran's deputy foreign minister, Saeed Khatibzadeh, said no date had been set for the next round of negotiations, adding that a framework of understanding must be agreed first.

Pressure for a way out of the war has mounted as Trump’s fellow Republicans defend narrow majorities in Congress in the November midterm elections with U.S. gasoline prices high, inflation rising and his own approval ratings down.

"The main thing is that Iran will not have a nuclear weapon. You cannot let Iran have a nuclear weapon, and that supersedes everything else," Trump said on Friday.

Trump also said he might end the ceasefire with Iran unless a long-term deal to end the war was agreed before it expires on Wednesday, adding that a U.S. blockade of Iranian ports would continue.

There were no signs of preparations early on Saturday for talks in the Pakistani capital, where the highest-level U.S.-Iran negotiations since the 1979 Islamic Revolution ended without agreement last weekend.

A Pakistani source aware of mediation efforts had said a meeting between Iran and the U.S. could produce an initial memorandum of understanding, followed by a comprehensive peace agreement within 60 days.

Separately, a senior Iranian official said Tehran hoped a preliminary agreement could be reached in the coming days.

Oil prices fell about 10% and global stocks jumped on Friday on the prospect of marine traffic resuming through the strait. Despite that, hundreds of vessels and about 20,000 seafarers remain stranded in the Gulf awaiting passage through the waterway, shipping sources said.

At last weekend's talks, the U.S. proposed a 20-year suspension of all Iranian nuclear activity, while Iran suggested a halt of three to five years, according to people familiar with the proposals. Two Iranian sources have said there were signs of a compromise that could remove part of the stockpile.

The head of Russia's state atomic energy company, Rosatom, Alexei Likhachev, said on Saturday that Rosatom was ready to help with the removal of enriched uranium from Iran, and that the company was closely following the progress of U.S.-Iran talks.

(Reporting by Reuters bureaus, Writing by Raju Gopalakrishnan, William Maclean and Timothy Heritage; Editing by William Mallard and Alex Richardson)