Syria has reacted with anger to a visit by the US ambassador to the besieged city of Hama, where a standoff between the army and protesters has gone on for five days.

Officials in Damascus claimed the visit by Robert Ford was a provocation, which added weight to its claims that the US was working to undermine President Bashar al-Assad and his regime.

"The presence of the US ambassador in Hama without previous permission is obvious proof of a clear evidence of the United States' involvement in current events in Syria and its attempt to incite an escalation in the situation, which disturbs Syria's security and stability," the Syrian foreign ministry said.

The US state department said in response that its embassy had advised the foreign ministry that diplomats would travel to Hama on Wednesday: "The fundamental intention was to make absolutely clear with his physical presence that we stand with those Syrians who are expressing their right to speak for change," it said.

Ford intends to stay in Hama until Friday – the first day of the weekend in Syria and the day on which most of the violence of the past four months has occurred. Ford's security convoy travelled through Syrian military checkpoints to reach Hama, and he is believed to have met residents and business owners inside the city centre.

The show of solidarity with protesters has added another dimension to the tension between Hama and the Syrian military, which seems unsure about how to deal with the long-rebellious city.

Another large rally has been planned in Hama a week after more than 200,000 people turned out for an anti-regime protest after Friday prayers. That mass show of dissent was the largest yet seen in the Syrian uprising.

Syrian officials have insinuated that Islamists are agitating among protesters with legitimate grievances and that the protest movement is being hijacked by Sunni militants bent on sectarian division. Hama residents have repeatedly derided the claims.

Protests continued in Damascus last night, with reports of three people killed inside a mosque in the Harasata area.

Rights groups report that more than 1,400 people have been killed during the violence. The Syrian government claims that more than 300 of its security forces have also been killed.