The letter was written by the Syrian National Youth Party, which opposes President Bashar al-Assad. The party was asking Putin to stop the fighting currently taking place in Aleppo.
"Ten days and nights in a row, Mr. President, the mouths of Aleppo children are filled with earth instead of milk, while the rubble of stones and mines are hitting their heads and bodies. Their mothers and fathers groan and scream from under the ruins, with tears in their eyes they're crying to stop this terrorism, which mankind hasn't seen in 7,000 years of history," Barvin Ibrahim wrote in the letter.
The letter also said that if Russia and the United States worked together, then peace in Syria would be quickly restored.
Last week, Moscow and Washington brokered a "silent" period for 24 hours in the Damascus suburbs and for 72 hours in the northern Latakia province, starting at midnight on April 30.
According to the latest information from the US State Department, Russia and the United States agreed to extend the "silent" period to the city of Aleppo.
The new "silent" period came into force at 00:01 AM local time on Wednesday.
The US is closely monitoring the new cessation of hostilities in Aleppo with Russia. "To ensure this continues in a sustainable way, we are coordinating closely with Russia to finalize enhanced monitoring efforts of this renewed cessation."
Washington counts on Moscow to put pressure on Damascus concerning the 'silent' period in the Aleppo province.
The State Department vows to continue efforts to expand the cessation of hostilities across Syria.
