Referring to Russia and China's February 4 veto of an Arab League peace plan, Pillay said that "the failure of the UN security Council to agree on firm collective action appears to have fueled the Syrian Government's readiness to massacre its own people in an effort to crush dissent."
Meanwhile, France and Britain have expressed doubts about Russian efforts to mediate in the Syrian crisis.
Pillay's statement was issued from Geneva on February 8 as the Syrian army reportedly intensified tank and artillery barrages on the fifth day of an assault on Homs.
The Nobel Peace Prize-winning Doctors Without Borders group has accused the Syrian authorities of using medicine "as a weapon of persecution."
The aid agency's president, Marie-Pierre Allie, said that "wounded patients and doctors are pursued and risk torture and arrest at the hands of the security services."
The agency said it was unable to work inside Syria because of the brutal suppression of revolt by Assad's forces, but it had interviewed witnesses who escaped to seek treatment in neighboring states. It quoted a Syrian doctor who said "security servives also attack and destroy mobile hospitals."
Reports say the offensive in Homs is targeting several Sunni Muslim neighborhoods that have risen up against the 11-year rule of President Assad -- a member of the minority Alawite community in Syria that has dominated the Sunni-majority country for the last five decades.
The London-based Syrian Observatory for Human Rights says at least 47 people were killed overnight in Sunni neighborhoods of Homs by heavy shelling and machine-gun fire from Syrian army troops.
The group also says three families were killed overnight by what it called regime-backed thugs.
Independent confirmation of the reports was not immediately available because of the Syrian regime's restrictions on foreign journalists.
But some foreign journalists working from within opposition strongholds in Homs this week have confirmed that government forces were launching heavy barrages of tank and artillery fire into Sunni residential areas of the city.
Activists in Homs say shelling has targeted four neighborhoods at the hub of antiregime protests -- Bayadah, Bab Amr, Khaldiyeh and Karm el-Zeytoun.
A spokesman for the opposition Local Coordination Committees says government troops at dawn on February 8 began storming Khalidiyeh, Bab Amr, and the neighborhood of Inshaat, seizing a hospital and arresting injured people there.
Spokesman Omar Idibi also said the advance by government troops was preventing injured civilians from reaching the hospital for treatment.
Other rights activists and opposition sources say Syrian army tanks had entered the Inshaat neighborhood and were moving closer to Bab Amr.
Omar Shaker, an activist in Bab Amr, says that neighborhood was under "very intense shelling" on February 8 by tanks, mortars, artillery, and heavy machine guns.
Rights activist Mohammad Hassan said by satellite phone from Homs that the Sunni Muslim neighborhood of Wadi al-Arab also was under attack. He also said tanks were deployed on the main thoroughfares of Homs.
According to Western media reports, the overnight death toll in Homs includes 18 prematurely born babies who died at a hospital because their incubators shut down as a result of electricity cuts. This could not be independently confirmed.
Compiled from agency and RFE/RL reports