All interested parties have responsibility for agreeing a ceasefire in Syria, but the regime of Bashar al-Assad is responsible above all, German Chancellor Angela Merkel said on Thursday.

Earlier, Merkel said at the start of a donor conference in London that Germany has pledged to give 2.3 billion euros ($2.57 billion) in aid to Syria by 2018, including 1.1 billion euros this year alone.

Meanwhile, Merkel's migration policies have caused her approval ratings to plummet to 46 per cent, the lowest reading since August 2011, a poll conducted on behalf of public broadcaster ARD showed.

Released late on Wednesday, the monthly Deutschlandtrends poll that surveys Merkel's public support showed a decrease of 12 percentage points from January.

A record 81 percent of respondents said that Merkel's government is no longer in control of the refugee influx that saw 1.1 million people enter the country in 2015.

Meanwhile, 63 percent of respondents favored a cap on the number of refugees entering the country in 2016, a suggestion that caused a controversy when it was first suggested by Merkel's Bavarian sister party, the Christian Social Union (CSU).

Merkel's waning popularity is also causing a shift in the political landscape, with her Christian Democrats (CDU) losing 4 percentage points since January, now polling at 35 percent.

The anti-immigrant Alternative for Germany (AfD) party gained 3 percentage points over the same period, making it the third strongest party in the country with 12 percent of votes.

Merkel has faced widespread criticism over her decision to keep Germany's borders open. An increasing number of politicians - many within her own coalition - are arguing for stricter border controls and speedy deportations for those not eligible for asylum.

Despite the shift in public opinion, the vast majority of Germans remain in favor of granting political asylum to refugees.

94 percent of respondents were in favor of welcoming refugees fleeing war, while 73 percent said Germany should welcome those facing political or religious persecution in their home countries.

Nearly three-quarters of respondents (71 percent), meanwhile, said that migrants fleeing poverty rather than conflict should not be granted asylum.