WASHINGTON/ISLAMABAD - 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 while trying to transit the waterway. In Mumbai, an Indian government source said India summoned Iran's ambassador after an Indian-flagged vessel carrying crude oil was attacked while trying to cross the Strait.

Tehran’s renewed tough messaging injected 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.

In Washington, 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 received radio messages from Iran’s navy saying no ships were allowed through the waterway, reversing signs earlier on Saturday that traffic might resume.

At least two vessels reported being hit by gunfire as they attempted to cross the strait, the sources said.

Earlier, maritime trackers had 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.

-Reporting by Reuters bureaus, Writing by Raju Gopalakrishnan, William Maclean; Editing by William Mallard and Alex Richardson/Reuters