Syria's state-run media have reported that a suicide bomber killed at least five people in central Damascus.
The reports said some 20 other people were wounded in the attack in Midan district, blaming "terrorists" for the blast.
Earlier on April 27, activists said a separate explosion was reported near a state-owned company in an industrial zone of Damascus but it was unclear if there were any casualties.
A wave of blasts has rocked Syrian cities in recent months, despite a diplomatic push to end an uprising against President Bashar al-Assad that began in March 2011.
Earlier, UN Secretary-General Ban Ki-moon warned that the Syrian government was not complying with the UN-backed peace plan to end more than a year of bloodshed.
In a statement, Ban said he was "deeply troubled" at reports Syrian heavy weapons were still located in population centers.
Ban said he was "gravely alarmed" at reports of continuing violence and killing in Syria, including shelling, shootings, and explosions in residential areas.
Call For More Monitors
Meanwhile, Arab League chief Nabil Elaraby has called for the quick deployment of UN monitors to Syria at a league meeting in Cairo.
Elaraby said he had been in contact with the international mediator, Kofi Annan, as well as UN chief Ban.
"This morning I spoke to Kofi Annan, and discovered that he is as distressed as I am. We agreed that I should send a letter to the UN secretary-general, Ban Ki-moon, and in fact I sent him a letter which I will distribute to you all," Elaraby said.
"In it, I stressed the importance of the swift deployment of monitors, which the Security Council has already decided on, and this should happen without delay."
There are now 11 observers in Syria, according to the UN.
Ban has said the first 100 of the 300 monitors should be on the ground inside Syria by the end of next month.
The Arab League also issued a statement calling on the UN Security Council to take immediate action to protect civilians in Syria.
Russia Urges Cooperation
At the United Nations, U.S. Ambassador Susan Rice reiterated that Assad's government has failed to live up to its commitments and said it was important for the Security Council to consider sanctions against Damascus.
In Moscow, Russia's Foreign Ministry urged the international community to make sure all sides in the conflict were held accountable for ending the violence.
"Currently, we see the main task in meticulous and strict implementation of the UN Security Council decisions on Syria that put responsibility on both the government and other Syrian sides for ending violence and for the necessity to cooperate with UN monitors and the mission of the special envoy, Kofi Annan," Foreign Ministry spokesman Aleksandr Lukashevich said.
"The real chance to stop the bloodshed and the suffering of civilians should be fully used."
Lukashevich also criticized a recent decision by the European Union to impose fresh sanctions on Syria due to the continued bloodshed despite the nearly two-week-old cease-fire.
"We still think that such single-sided sanctions are counterproductive and call on the international community to concentrate on supporting the activities of UN and Arab League special envoy Kofi Annan," Lukashevich said.
With reporting by AP, Reuters, and AFP