Saudi Arabia and Turkey have attacked the Syrian regime and Russia for the targeting of rebel leaders in air raids.
The outburst follows last week's killing of Zahran Alloush, leader of Syrian rebel group Jaish al-Islam.
Adel al-Jubeir, Saudi Arabia's foreign minister, is known to be a hard-line opponent of Syria's President Bashar al-Assad, while Riyadh was seen as Jaish al-Islam's key backer.
Jubeir said Alloush's killing was detrimental to the war against the Islamic State group.
"Attempts to assassinate leaderships fighting [IS] do not serve the peace process or [efforts] to achieve a political solution in Syria," Jubeir remarked.
"I don't know what the Russians have in mind," he added.
|Our views on Syria match. We cannot reach a solution with Assad's presence
- Mevlut Cavusoglu
His Turkish counterpart, Mevlut Cavusoglu, agreed.
"Our views on Syria match. We cannot reach a solution with Assad's presence," he said.
Alloush led Jaish al-Islam, a rebel group based in the heavily bombed Damascus suburbs of Ghouta.
The armed movement had proved one of the most formidable rebel groups in the Syria war, and Alloush was considered a charismatic and capable leader.
Jaish al-Islam is also at war with IS, and the group was key in flushing out extremists from large parts of a Damascus Palestinian refugee camp earlier this year.
Alloush also agreed to take part in peace talks with the Syrian regime, after opposition groups met in Riyadh this month to hammer out a joint negotiating team.
With the deaths of potential opposition negotiators, allegedly at the hands of the Syrian regime and Russia, many analysts believe there is an organised campaign to target "strongmen" in the opposition camp before talks begin.
Russia has also been heavily bombing Turkish-backed rebels and the Free Syrian Army in the north of Syria, who were also supposed to take part in the peace talks.
Saudi Arabia and Turkey have grown closer over their mutual agreement on backing rebel groups in Syria's war.
Riyadh and Ankara have also found common adversaries who back the Syrian regime, such as Iran - and now Russia, after Turkey shot down a Russian fighter jet in November.
The incident led to a major diplomatic row between Turkey and Russia, with Moscow accusing Ankara of supporting the Islamic State group and buying its oil illegally.
|If you are really willing to confront IS, then let us agree and eliminate it
- Mevlut Cavusoglu
Turkey maintains that the vast majority of Russian air raids hit civilian areas and rebel groups at war with IS - and this includes the attack that killed Alloush.
"If you are really willing to confront IS, then let us agree and eliminate it," Cavusoglu said. He accused Russia of having "goals other than eliminating IS".
Turkey's President Recep Tayyip Erdogan arrived in Saudi Arabia on Tuesday for talks over "cooperation in confronting terrorism and radicalism" with King Salman al-Saud.
Riyadh recently announced the formation of a 34-nation military coalition of Muslim-majority states against "terrorism".
Many suspect this will be central to establishing a new order in the Middle East.
Although most member states have been struggling to deal with a growing extremist threat from IS sympathisers, many suspect the real aim of the "Sunni club" is to take action against the regional powers not included in the coalition - Iran, Iraq and Syria.
With Turkey and Saudi Arabia having probably the largest and most advanced militaries in this coalition, it would undoubtedly be these two powers who lead the formation.
Jubeir and Cavusoglu spoke on Tuesday of a "willingness" to form a joint strategic coordination council, to find a united front on issues of security and regional harmony.