President Donald Trump on Thursday accused Iran of doing a "very poor job" of allowing oil through the Strait of Hormuz and of breaching the terms of their two-week ceasefire agreement.
In a barrage of social media posts that sparked fresh fears for the shaky truce, Trump also warned Iran against imposing a toll for ships passing through the crucial waterway.
"Iran is doing a very poor job, dishonorable some would say, of allowing Oil to go through the Strait of Hormuz," Trump said on his Truth Social platform.
"That is not the agreement we have!"
Iran and the United States said the 167-kilometer (104-mile) strait between the Gulf and the Indian Ocean would reopen after the two-week truce was announced on Tuesday.
But just 10 vessels have passed through since the Middle East war ceasefire took effect, according to maritime tracking data.
Tensions have risen further after Iran suggested imposing a toll on ships -- though Trump has made similar suggestions and even aired joint tolls with Tehran.
"There are reports that Iran is charging fees to tankers going through the Hormuz Strait -- They better not be and, if they are, they better stop now!" Trump said in an earlier Truth Social message.
In yet another post in which he raged at a critical media editorial on the ceasefire, Trump added that "very quickly, you'll see Oil start flowing, with or without the help of Iran."
The U.S. leader's tone appeared to darken noticeably from earlier comments to NBC News in which he said he was "very optimistic" about a peace deal with Iran after their ceasefire, and that Israel was "scaling back" strikes in Lebanon.
Trump told the broadcaster in a telephone interview that Iran's leaders were "much more reasonable" in private but added that "if they don't make a deal, it's going to be very painful."
Vice President JD Vance is due to hold talks with Iran in Pakistan on Saturday. Special envoy Steve Witkoff and Trump's son-in-law Jared Kushner are also traveling to Islamabad.
"The president is optimistic that a deal can be reached that can lead to lasting peace in the Middle East," White House Principal Deputy Press Secretary Anna Kelly told AFP in a statement on Thursday.
Israeli Prime Minister Benjamin Netanyahu had meanwhile agreed in a call with Trump on Wednesday to "low-key it" with Lebanon after devastating strikes, the U.S. president said.
Israel and Lebanon will hold talks in Washington next week, a State Department official said Thursday.
Israel's heaviest strikes on Lebanon since Hezbollah entered the Middle East war in early March killed hundreds on Wednesday, rattling the uneasy truce between Washington and Tehran less than 48 hours after it came into force.