Global oil prices climbed as doubts intensified over the durability of a fragile ceasefire between the United States and Iran, following a new wave of Israeli airstrikes in Lebanon.

Tehran responded by warning of a "regret-inducing response" should the attacks persist, while US President Donald Trump said American forces would remain deployed in the region until Iran adheres to what he called the "real" ceasefire terms.

Prices had fallen sharply on Wednesday after the truce was announced, alongside plans to reopen the strategically vital Strait of Hormuz.

However, renewed concerns emerged after reports indicated Iran may keep the critical shipping corridor closed in response to Israeli military action, raising the prospect of prolonged disruptions to global energy flows.

Brent crude, the international benchmark, rose 2% to $96.53 per barrel, while US West Texas Intermediate gained 2.8% to $97.02, as what Vice President JD Vance described as a "fragile truce" came under pressure.

Equity markets also gave up some of their earlier gains. Japan’s Nikkei 225 slipped 0.5%, and South Korea’s Kospi dropped 1.8%.

"I think there's a little bit of nervousness in global markets," Victoria Scholar, head of investment at Interactive Investor, told the BBC's Today programme.

"Markets are giving back some gains… and I think that reflects a lot of uncertainty over whether the Strait of Hormuz is actually open."

Sim Moh Siong, a strategist at Singapore-based OCBC, said attention will center on whether energy shipments can continue through the strait, as questions remain about how Tehran intends to manage vessel traffic.

A key provision of the ceasefire required that ships be allowed to pass safely through the Strait of Hormuz.

Yet vessels operating in the Gulf have reportedly been warned by Iran’s navy that any attempting to transit the strait without authorization "will be targeted and destroyed," according to shipping brokerage SSY, which confirmed the message to BBC Verify.

Since the agreement was announced, only a limited number of ships have crossed the passage—far fewer than the roughly 130 vessels that moved through it daily prior to the conflict.

Maritime tracking firm Pole Star Global estimates it would take at least 10 days to clear the current backlog, even if normal traffic resumes.

Several nations, including Malaysia, India, and the Philippines, have recently secured arrangements to allow their ships safe passage.

"It is very difficult to plan because every day you get very different news," said Nils Haupt from container shipping firm Hapag-Lloyd, which still has six ships in the Persian Gulf.

"Yesterday [Wednesday] was a typical day. You get the news, 'it will now be open and something is happening now', and then in the evening it is not happening," he told the Today programme.

Haupt added that uncertainty also surrounds potential transit fees for the strait, cautioning that any significant charges could have wide-reaching consequences.

"If this means that for the coming years there will be a fee for the Strait of Hormuz of millions which is double, triple the price of crossing the Panama Canal or the Suez Canal it would be quite ridiculous for the entire industry."

Questions also persist about whether Lebanon is covered under the ceasefire terms.

On Wednesday, Israel carried out its most intense strikes of the conflict in Lebanon, with at least 182 people reported killed.

Hezbollah said in a statement posted on social media that it had fired rockets at northern Israel. The Iran-backed militia says it was in response to ceasefire violations.

Vance is expected to participate in talks with Iran in Pakistan on Saturday.