Syrian activists say government forces bombarded Aleppo Sunday, reportedly killing dozens of people and helping exacerbate an ongoing water shortage in the country's biggest city.

Opposition activists say the attacks targeted residential areas in the northern city of Aleppo. Earlier, rebel forces had been trying to seize the areas from the government.

The exact number of casualties is unclear, and the reports have not been independently verified.

The violence comes a day after water supplies to residents in Aleppo were cut after a major water pipe was damaged during intense fighting between government forces and rebels.

Opposition activists say the pipeline was hit as Syrian forces shelled rebel targets, while Syrian officials accuse the rebels of sabotage.

Meanwhile, U.S. Secretary of State Hillary Clinton says discussions with Russian leaders about Syria have not been productive.

The top U.S. diplomat was in Russia for a summit of Pacific Rim countries. She said her talks Sunday with Russian President Vladimir Putin and Foreign Minister Sergei Lavrov made little progress on how to deal with the Syrian rebellion against President Bashar al-Assad's autocratic rule.

Clinton said if the differences persist, she is willing to work with “like-minded” states to support the Syrian opposition in its struggles.

Lavrov said unilateral U.S. sanctions against Syria appear more and more extra-territorial and have a direct impact on Russian business interests. He said Moscow has made it clear that this approach is not acceptable.

Russia and China have repeatedly blocked attempts by Western and Arab powers to pass U.N. Security Council resolutions threatening sanctions against President Assad.

The 18-month-old uprising in Syria has left 20,000 people dead and forced thousands to flee from their homes.