The United Nations says at least 2,600 people have been killed in Syria during the anti-government uprising that has swept the country since mid-March.

U.N. High Commissioner for Human Rights Navi Pillay provided the figures during an address to the Human Rights Council on Monday. The figure represents a significant increase since August, when the U.N. estimated that 2,200 had died since the government began violently cracking down on dissent.

An aide to Syrian President Bashar al-Assad was quick to counter the U.N. figures Monday, saying that only 1,400 have died. Syrian government spokesman Bouthaina Shaaban says the casualties have been evenly split, with 700 government forces and 700 opposition activists having been killed.

Meanwhile, the U.S. has condemned the killing of prominent Syrian human rights activist Ghiyath Mattar, who died last week while in the custody of Syrian security forces. State Department spokeswoman Victoria Nuland denounced what she called the “despicable violence” of the Syrian regime, again reiterating the U.S. call for President Assad to step down.

Mr. Assad has been facing growing international pressure to curb the violence. Last week, Russian President Dmitry Medvedev indicated Moscow may be willing to consider a U.N. Security Council resolution that sends a “strong message” to Mr. Assad.

However, during a meeting with British Prime Minister David Cameron on Monday, Mr. Medvedev said there is no need for “additional pressure” on Syria, saying the international community has already applied enough sanctions against the Assad regime.

On Friday, European Union diplomats said the group had neared an agreement on sanctions barring new investments in Damascus' energy sector. A week ago, the EU announced economic sanctions that include an oil embargo.