The US ambassador to Damascus was in Hama today to express "our deep support for the right of the Syrian people to assemble peacefully," State Department spokesman Victoria Nuland said in Washington.
A senior US official, speaking on condition of anonymity, said Ambassador Robert Ford visited the tense city "to make contact" with opposition leaders.
Syrian authorities have been trying to quell protests in Hama, traditionally a centre of opposition to central government, and had positioned tanks on the main entrances to the city except in the north.
The Syrian Observatory for Human Rights said that about 100 families -- or 1,000 people in total -- had left Hama, where it said Syrian troops had killed 23 civilians since Tuesday.
The crowds leaving Hama were headed for Salamiyah, some 30 kilometres (18 miles) to the southeast.
Ammar Qorabi, head of the National Organisation for Human Rights, said on Wednesday there had been a worsening of the security situation with the "pursuit of search operations, assassinations and arrests in this city."
Hama has been a symbol of opposition since the 1982 crackdown on a revolt by the banned Muslim Brotherhood against then-president Hafez al-Assad, father of the present leader, in which some 20,000 people were killed.