The rebel Free Syrian Army says it will observe a cease-fire for the Muslim religious holiday of Eid al-Adha, which begins October 26.
But a leading commander, General Mustafa al-Sheikh, said from Turkey that if the government breaks the cease-fire, his fighters will do so, too.
The Syrian military announced earlier it will cease military operations against rebels from October 26 until October 29 during Eid al-Adha.
The Syrian military said it reserves the right to respond to rebel attacks and bombings.
The cease-fire has been pushed by the UN in the hope that it will lead to talks between the warring parties and an end to fighting in the country.
Martin Nesirky, a spokesman for the UN Secretary-General Ban Ki-moon, welcomed the announcement and said it was paramount that the Syrian government and opposition groups adhere to the truce.
"We all understand that there is a lack of trust between parties and therefore we all understand that we cannot be sure yet what will transpire, but the hope is that the guns will fall silent," Nesirky said.
'World Is Watching'
The UN spokesman said the "world is now watching" to make sure the Syrian government and rebels observe a cease-fire.
He added, "We would simply fervently hope that the guns do fall silent, that there is a suspension in the violence so that humanitarian workers can help those who are most in need."
The United States also expressed hopes the cease-fire will be observed by all sides.
State Department spokeswoman Victoria Nuland told reporters in Washington, "What we are hoping and expecting is that they will not just 'talk the talk' of cease-fire, but that they will 'walk the walk,' beginning with the regime, and we will be watching very closely."
Meanwhile, fierce fighting raged in Aleppo as Syrian rebels claimed to have seized two districts of the city.
Activists in Aleppo say the heaviest clashes are around a military airport.
If the rebel gains are confirmed, the advances would mark a change in the stalemate that has characterized fighting in the city since it began in July, with both sides largely entrenched on the frontlines.
Based on reporting by Reuters and dpa