Britain condemned continued violence in Syria despite the presence of Arab League observers and called for an immediate end to state repression of protests.
At least 130 people are reported by opposition groups to have been killed by regime forces since the mission began its work on Tuesday.
Security forces have opened fire on large demonstrations in several areas of the country, including parts of the capital Damascus, where the observers were expected.
Demonstrations against the government of President Bashar al Assad are expected to escalate later - with calls for mass marches to main squares after Friday prayers.
Foreign Office minister Alistair Burt discussed the latest developments in a telephone call last night with the League's general secretary Nabil Elaraby.
"The UK has consistently supported the Arab League's efforts to encourage the Assad regime to end the cycle of violence in Syria," he said.
"I welcomed the Arab League's plans to increase the size of the mission over the coming days, which will bolster its capacity to report independently on the situation in Syria.
"I look forward to the mission's interim report in due course."
He went on: "Unfortunately, reports show that the violence has continued in Syria over the past few days. I urge the Syrian government to meet fully its obligations to the Arab League, including immediately ending the repression and withdrawing security forces from cities.
"The Syrian government must allow the Arab League mission independent and unrestricted access to all areas of Syria and to the Syrian people."