"It is very regrettable that Russia and China have refused for some time to come to a common position (with Western partners) on the Syrian conflict. This considerably weakens the role of the United Nations," she said in an interview with regional daily Augsburger Allgemeine.
Russia and China have vetoed three resolutions that would increase pressure on Syrian President Bashar al-Assad since the start of the conflict in March
The two countries are also against a current push by the three Western permanent members of the Security Council -- the United States, Britain and France -- for a resolution that would allow military action against Syria over a chemical weapons attack which the West blames on the regime and the regime blames on the rebels.
In a separate interview to be published on Sunday in Welt am Sonntag weekly, Foreign Minister Guido Westerwelle urged Russia to reconsider its stance.
"Those who look away despite the use of chemical weapons ... encourage their use. That is why we are calling on Russia to send a signal along with the international community," he said.
Merkel reiterated that the use of chemical weapons in Syria had "broken a taboo" which "cannot remain without consequence."
However, she categorically excluded German participation in any military action without prior approval from the international community.
"Germany cannot participate in any military intervention without a mandate from the United Nations, NATO or the EU," she said.
"Therefore, there is no question of any participation by the Federal Army at the moment," added Merkel, who is seeking to be elected again for a third consecutive term in a September 22 vote.
A poll published Thursday showed German public opinion firmly against military action by the West in Syria, with about six in ten people opposed to any possible strikes and only about a third in favour.