Syrian opposition says it will boycott Syria peace talks being held in Astana, demanding a complete end to regime bombardment on areas under rebel control.
Syrian activists say a large explosion in a northern town along the border with Turkey has killed at least five people and wounded many others.
The Britain-based Syrian Observatory for Human Rights (SOHR) says the death toll in the opposition-held town of Azaz is likely to climb after Wednesday's blast.
Azaz is about 7 kilometres from Turkey's border town Kilis. It has long been an important base for the opposition, including groups backed by Turkey.
The explosion happened near the building of the Syrian Interim Government which represents the opposition in rebel strongholds.
Both the SOHR and the activist-run Revolutionary Forces of Syria media office say the explosion is believed to have been caused by a car bomb. Images posted online by the activist-run Azaz Media Centre showed a burned out car and a fire in the area.
There was no immediate claim of responsibility for the blast.
Syrian opposition boycotts current round of Astana talks
The Syrian opposition said on Wednesday it had suspended its participation in the talks on settling the Syrian crisis being held in Astana, Kazakhstan. It has demanded an end to regime bombardment of the areas under its control.
"The delegation has suspended its participation after presenting a memorandum for a total commitment to stopping (regime) bombardments," political opposition SNC spokesman Ahmad Ramadan said.
However, the status of the talks was unclear as Kazakhstan's foreign minister said he expected the opposition to continue the peace talks on Thursday.
Wednesday had been set aside for bilateral meetings.
"Today (we) are having bilateral meetings. So we'll meet everyone in order to be better prepared for tomorrow, so I am not making any further comments except that today is the day for bilateral meetings with every observer and delegation," UN Syria envoy Staffan de Mistura told media earlier as he arrived for the talks, which are scheduled to end on Thursday.
Russian Foreign Minister Sergey Lavrov told news agencies on April 29 that Moscow was ready to cooperate with the United States on settling the Syrian crisis, and the White House said Washington would send a representative to the Kazakh capital.
Media also quoted Russian Deputy Foreign Minister Mikhail Bogdanov as saying the armed Syrian opposition would participate in the peace talks on May 3 and 4, which will also involve Iran and Turkey.
The Syrian opposition boycotted the Astana talks in March that took place as a deadly double suicide bombing marked the start of the civil war's seventh year. The meetings ended without substantive negotiations taking place.
The only concrete outcome from the third round of talks in Astana was an agreement between Turkey, Russia and Iran, who are sponsoring the negotiations, to meet again in early May.
Putin and Trump on Syria
US President Donald Trump and Russian President Vladimir Putin on Tuesday discussed working together to end the violence in Syria in their first phone call since US air strikes in Syria strained US-Russian relations.
The White House said the two leaders agreed that "all parties must do all they can to end the violence" in Syria.
Trump's decision to launch 59 cruise missiles against a Syrian airfield on April 4 in retaliation for a regime chemical weapons attack angered Russia, a key ally of regime leader Bashar al Assad, and led to some heated words between Washington and Moscow.
Trump and Putin also discussed working together against militants throughout the Middle East.
Increasing death toll
The death toll in a Daesh attack on Tuesday in an area held by the US-backed Syrian Democratic Forces (SDF) in northeast Syria rose to at least 38 people. This included 23 civilians, SOHR said.
The SDF is dominated by the YPG, the armed wing of the PYD, a Syrian affiliate of the PKK, which Turkey, the US and EU list as a terrorist organisation.
The attack on Rajm al Salibi, the location of a checkpoint and refugee camp near the border with Iraq, led to fierce clashes in Hasakah province, injuring dozens, the Britain-based war monitor said.
The International Rescue Committee said the wounded were taken to a hospital inside Hasakah and several children were killed.
Thousands of people have fled Iraq for the Rajm al Salibi border crossing and another camp further north in recent months, the global aid agency said.