British Prime Minister David Cameron has said the use or threat of chemical weapons by the Syrian government "was completely unacceptable," echoing warnings from U.S. President Barack Obama.
On August 21, Obama declared the threat of chemical or biological warfare in Syria a "red line" for the United States.
Cameron's office also said Britain, France and the United States have discussed ways to bolster Syria's opposition in their 17-month struggle again President Bashar Assad.
Earlier, Russia's Foreign Ministry accused Western powers of "openly instigating" the Syrian opposition to take up arms against Assad.
The UN's political chief Jeffrey Feltman, meanwhile, told the UN Security Council that Iran's arms delivery to Syria was an apparent violation banning Iranian arms exports.
"The Syrian people are suffering grievously from the appalling further militarization of this conflict. The Secretary-General has repeatedly expressed his concern about the arms flows to the two parties in Syria, which in some cases appear to violate resolution 1747 passed by this council banning arms exports under Chapter 7 authority," Feltman said.
Resolution 1747 bans Iran from exporting arms. The authorization comes under Chapter 7 of the UN charter, which allows the Security Council to authorize actions ranging from diplomatic and economic sanctions to military intervention.
After the Security Council meeting, UN aid chief Valerie Amos described the humanitarian situation in Syria.
"I visited Syria and Lebanon last week to see for myself the impact of the intensifying conflict and to discuss ways to increase humanitarian assistance to the men, women and children who are facing the terrible effects of the crisis. The humanitarian situation has worsened since my visit in March," Amos said.
"According to the government's own figures, 1.2 million people are sheltering in public buildings, many more are staying with relatives and friends. Both those who have fled and their hosts have urgent humanitarian needs due to the widening impact of the crisis on the economy and on people's livelihoods," she added.
Inside Syria, on August 23, government troops -- backed by tanks, helicopters, and artillery -- continued their offensive for a second day on the southern edges of Damascus.
The assault is the heaviest violence around Damascus since the army claimed it had retaken most of the capital from rebels last month.
Dozens of deaths were reported on August 23 from the assault on the mostly Sunni Muslim suburb of Daraya and in the capital's southern district of Al-Hajar Al-Aswad.
Activists say President Bashar al-Assad's forces also raided the southeastern Kafr Souseh area early on the same day, making many arrests.
Elsewhere, in the northern city of Aleppo, residents say government forces moved back into three predominantly Christian neighborhoods that had been under the control of rebel fighters since the weekend -- Jdeide, Telal and Sulamaniyeh.
Based on AP, Reuters, AFP, and dpa reporting