Tensions flared between the United States and Syria on Friday after the U.S. ambassador in Damascus appeared in the city that has become the focal point of protests against the government of President Bashar al-Assad.

Ambassador Robert Ford’s sport-utility vehicle was surrounded in Hama on Friday by cheering demonstrators who waved olive branches and tossed pink and red flowers onto the hood, according to State Department officials and a video posted on YouTube.

Syria’s state-run news agency, quoting an “official source” at the Foreign Ministry, said Friday that the ambassador’s presence in Hama “without obtaining prior permission from the Foreign Ministry . . . is clear evidence of the U.S. involvement in the ongoing events in Syria.”

State Department spokeswoman Victoria Nuland said the embassy had informed Syria’s Defense Ministry in advance of the visit. “This was not about us getting in the middle of it,” she said.

Nuland acknowledged that Ford’s trip “was a bold thing for him to do. But it speaks to the importance of sending the signal that we stand with the Syrian people.”

In another development, the State Department said Friday evening that it had called in Syrian Ambassador Imad Mustapha this week to complain that his personnel had been doing video and photo surveillance of peaceful demonstrators in the United States.

The U.S. government “takes very seriously reports of any foreign government actions attempting to intimidate individuals in the United States,” the statement said. It also said the department was investigating reports that the government in Damascus had retaliated against Syrians whose relatives in the United States were involved in protests.

The U.S. government has carefully calibrated its criticism as the Syrian rebellion has grown, leaving more than 1,300 dead, according to activists.

Washington has not called for Assad’s departure, as it did with Libyan leader Moammar Gaddafi. But in recent weeks it has imposed new sanctions on Assad and his inner circle, and the latest actions marked an escalation in tensions.

The U.S. government has long had a difficult relationship with Syria because of its ties to Iran, but Washington also is wary of the overthrow of a government that has held together a diverse population and has been seen as a key player in the Middle East.

Richard Murphy, a former U.S. ambassador to Syria, said that Ford was doing his job in gathering information in Hama. But Ford was undoubtedly aware that his trip “could become a club in the hands of the government to vindicate its stand that it will always resist foreigners,” who are often blamed by Syrian officials for the country’s problems, he said.

Hama has been surrounded by tanks in the wake of a large demonstration on July 1.

The visit by Ford, and a separate appearance in Hama by the French ambassador, appeared to embolden the protesters. Hundreds of thousands turned out for a fresh demonstration in the city of 700,000 on Friday afternoon.

Ford, however, did not remain in the city to witness it, Nuland said.

Hama is famous as the site of an Islamist uprising in 1982 that was brutally suppressed by President Hafez al-Assad, Bashar’s father. At least 10,000 people died.