"My request is that we get as quick as possible the results of the inspection team, that we have an independent statement by an independent neutral institution," he said on arriving in the Lithuanian capital for European Union foreign ministers' talks.
Westerwelle said that on the basis of information gleaned by British, French and US intelligence, "it is plausible that chemical weapons have been used in Syria" and also plausible that responsibility for the use of the weapons outside Damascus on August 21st lay with President Bashar al-Assad's regime.
But some key world powers had doubts, he said. "Therefore I think it is a good way to ask our colleagues in the United Nations to speed up with the exploration of the inspection team," he said. "I think that's important for many other discussions in some capital cities."
On Thursday Westerwelle said that the International Criminal Court (ICC) should examine the use of chemical weapons in Syria.
"I have again pressed for the UN Security Council to give a mandate to the ICC to examine the chemical attack in Syria," he said on the sidelines of the G20 summit in Russia's Saint Petersburg.
"We have launched this initiative already in January," he said. "The situation has worsened sufficiently for another initiative."
"Naturally, use of chemical weapons is absolutely unacceptable," said the foreign minister.
"We will also discuss this with my European colleagues in Vilnius" at the meeting of European foreign ministers on Friday and Saturday, he added.
European countries are attending the high-pressure summit where the Syrian conflict has overshadowed the economic agenda.
Any action against the Syrian regime's alleged use of chemical weapons through the UN Security Council has been so far ruled out by opposition from permanent veto-wielding members Russia and China.