UNITED NATIONS (Reuters) - U.N. Secretary-General Ban Ki-moon ratcheted up the pressure on Syrian President Bashar al-Assad on Thursday to end his 15-month onslaught against an opposition seeking to oust him, condemning a reported new massacre in Syria as an "unspeakable barbarity."
Ban and international mediator Kofi Annan both criticized Assad for failing to implement Annan's six-point peace plan, and Ban later told the U.N. Security Council it may be time to reassess the U.N. observer mission, which has done little to stop the violence.
"Today's news reports of another massacre in (Mazraat) al-Qubeir ... are shocking and sickening," Ban told a special session of the 193-nation General Assembly.
"We condemn this unspeakable barbarity and renew our determination to bring those responsible to account," he said.
Ban said U.N. monitors were initially denied access to the site, where opposition activists say at least 78 people were killed on Wednesday. "They are working now to get to the scene," he said. "And I just learned a few minutes ago that while trying to do so the U.N. monitors were shot at with small arms."
Annan, speaking after Ban, condemned the Mazraat al-Qubeir massacre and acknowledged his peace plan was not working. He suggested that those who fail to comply with the peace plan should be penalized, a remark the United States and its European allies will likely welcome.
"As we demand compliance with international law and the six-point (peace) plan, it must be made clear that there will be consequences if compliance is not forthcoming," Annan said.
Later, in the closed-door Security Council session, Annan warned the Syria crisis would soon spiral out of control and called for "substantial pressure" on Damascus and consequences for undermining the peace effort, U.N. diplomats said.
Ban said hopes for success of Annan's bid were fading and suggested that the United Nations must soon decide whether its 300-member monitoring mission in Syria was sustainable or useful, the diplomats said, speaking on condition of anonymity.
It was the first time the U.N. leader has suggested pulling the plug on the monitoring mission, which the international community had hoped could help end 15 months of violence that have brought Syria to the brink of civil war.
Russia and China, which have veto powers on the U.N. Security Council, have made clear they would strike down any attempts to impose sanctions on Damascus.
The Syrian opposition and Western and Gulf nations seeking Assad's ouster increasingly see Annan's peace plan as doomed due to the Syrian government's determination to use military force to crush an increasingly militarized opposition.
RUSSIA CONTINUES TO PROTECT ASSAD
Annan, contrary to expectations, did not bring up a proposal under which a new contact group would have been formed involving Russia, China, the United States, Britain, France and key regional players with influence on Syria's government and the opposition, such as Saudi Arabia, Qatar, Turkey and Iran.
Western officials, who blame Iran for backing Assad, reacted negatively to the idea of adding Tehran to the group, and diplomats said Annan may have decided to pull back for now.
Russian Ambassador Vitaly Churkin, however, reiterated that Moscow wanted to organize a meeting with the permanent council members, Iran and other regional players, diplomats said.
Thursday's U.N. session appeared to fall short on new ideas for winning Russian support for a Syrian political transition, which remains the thrust of Annan's peace plan and which both the Syrian government and opposition said they accepted earlier this year but have failed to implement.
Syria's U.N. ambassador, Bashar Ja'afari, took the floor at the General Assembly to respond to the condemnations of his government but the transmission of his rebuttal for those not permitted into the hall was cut off for several minutes.
Ja'afari told the assembly the new reported killings were "truly an atrocious massacre. It is unjustifiable." The Syrian government has blamed recent atrocities on the opposition and Islamist extremists it brands as terrorists.
Speaking earlier to the assembly, Churkin warned countries arming Syria's opposition that such arms may end up in the hands of "terrorists" and said that an investigation of the latest massacre should be undertaken. Assad's ally Russia is Syria's principal arms supplier and has a key naval port in Syria.
The representative of Iran, Assad's other main ally, hinted that a full civil war in Syria could spread beyond its borders.
"There is ... an urgent need for all those who are providing money, logistics and arms to the opposition groups to stop that immediately. They should know that the smoke from the fire in Syria will, in one way or another, cover the region as a whole," Iranian Ambassador Mohammad Khazaee told the assembly.
The conflict has already sparked turmoil in Lebanon and on the border of Turkey. Syria accuses Qatar and Saudi Arabia of arming the opposition, while the United Nations has said some bombings appeared to have been the work of extremist militants.
(Editing by Will Dunham)