* U.S. says burden on Syrian government to fulfill peace plan
* Annan due to brief U.N. Security Council on Thursday
UNITED NATIONS, April 11 (Reuters) - The United States dismissed Syria's new pledge to halt all fighting by dawn on Thursday, saying it holds "little if any credibility" and that caveats placed by President Bashar al-Assad's government on the ceasefire are worrying.
U.S. Ambassador to the United Nations Susan Rice said the commitment - made by Syria in a letter to U.N. and Arab League envoy Kofi Annan - could not be construed as complying with the former U.N. secretary-general's peace plan.
"The burden remains squarely on the Syrian regime and not the opposition in the first instance to meet its obligations in full and visibly under the Annan plan," Rice told reporters.
"The caveats in the letter are worrying and yet again cast into doubt the credibility of any such commitments but nothing casts more doubt on the credibility of the commitments than the fact that commitments have been made and made and made and broken and broken and broken," she said.
In the letter to Annan on Wednesday, Syria said it reserves the right to respond to any attack by "armed terrorist groups."
Damascus had agreed to a U.N. Security Council-backed deadline of Tuesday to withdraw troops Syrian towns and stop using heavy weapons. That missed deadline was supposed to be followed by a full ceasefire by the Syrian army and opposition forces on Thursday morning, but fierce fighting has continued.
"Fighting is still raging as we speak, reflecting what has been an intensification of the violence that the Syrian government has pursued since April 1 when it committed to cease all hostile actions by yesterday," Rice said.
"Its commitments, therefore, have little if any credibility given that track record."
The United Nations says more than 9,000 people have been killed by Syrian forces since March 2011. Damascus says opposition rebels have killed more than 2,500 soldiers and security personnel.
Annan is due to brief the U.N. Security Council on Thursday.
Any action by the council would need the support of Russia and China, which have blocked previous Security Council draft resolutions on Syria, citing concerns about a Libya-style intervention that would breach Syrian sovereignty.
Annan's judgment on whether Syria had complied with his plan would be key in determining the council's next move, said a diplomat on the council, who declined to be identified.
"But it would be totally inadequate if they (the Syrian government) don't withdraw from population centers but keep troops and heavy artillery in place, ready to pound the population at whatever they unilaterally judge to be a provocation," said the diplomat.