Russia said Wednesday it would press ahead with its bombing campaign in Syria until "terrorist organizations" such as the Islamic State group are defeated amid the latest international push to end the conflict.

"Russian air strikes will not cease until we truly defeat the terrorist organizations ISIL and Jabhat al-Nusra," Russian Foreign Minister Sergei Lavrov told reporters in Oman, referring to the IS and Al-Nusra Front jihadist groups, Interfax news agency reported.

World leaders gather in London on Thursday to try to raise $9 billion (8.3 billion euros) for the millions of Syrians hit by the country's civil war and a refugee crisis spanning Europe and the Middle East.

The donor conference, the fourth of its kind, hopes to meet the United Nations' demand for $7.73 billion to help in Syria plus $1.23 billion assistance for countries in the region affected by the crisis.

The latest push to end Syria's tangled conflict was in fresh disarray Tuesday as the government denied formal talks had begun and the opposition canceled a meeting with the U.N. envoy following intense Russian air strikes.

The main opposition umbrella group attending the biggest effort yet to end the war called Russia's bombardment near Aleppo -- with 270 raids since Monday morning, according to monitors – "unprecedented."

Britain said Monday that the U.N.-brokered peace talks on Syria being held in Geneva must lead to a political "transition away" from President Bashar Assad.

"Syrian peace negotiations must deliver a political transition away from Assad and end the suffering of the Syrian people," British Foreign Secretary Philip Hammond said in Amman at a joint news conference with his Jordanian counterpart Nasser Judeh.

Eight people reportedly died last month from a lack of proper medical care in a besieged Syrian town that was the site of a 2013 chemical weapons attack, the U.N. has said.

In a Sunday update, the U.N.'s Office for the Coordination of Humanitarian Affairs (OCHA) reported a "sharp deterioration of the humanitarian situation" in Moadimayet al-Sham, southwest of Damascus.

The United Nations human rights chief on Monday said that no amnesty should be possible for people suspected of committing war crimes or crimes against humanity, as Geneva hosted troubled talks aimed at ending Syria's war.

"We do have a principled position in the United Nations that no amnesties should be considered for those suspected of having committed crimes against humanity or war crimes," U.N. rights chief Zeid Ra'ad al-Hussein told reporters in Geneva.

Representatives of Syria's main opposition body arrived in Switzerland Saturday for U.N.-organised peace talks as the starvation death toll rose in Madaya, one of a string of besieged towns in the war-ravaged country.

The High Negotiations Committee (HNC) begrudgingly bowed only late Friday to U.S. and Saudi pressure to at least show up in Geneva to test the waters for joining the biggest push yet to end a five-year-old civil war.

Bowing to U.S. demands, the Dutch government announced Friday it will take part in U.S.-led air strikes in Syria against the Islamic State group, extending its current air support mission over Iraq.

"In order to make the fight against ISIS in Iraq more efficient, it has been decided to carry out air strikes against ISIS in eastern Syria," the foreign and defense ministries said in a statement.

Iranian President Hassan Rouhani has voiced pessimism over the prospects for peace in Syria, ahead of talks set to open in Geneva on Friday.

"It's our hope to see these negotiations succeed as quickly as possible," Rouhani told French media late Thursday following a two-day visit.

The percentage of Syrian refugees arriving in Greece has fallen considerably in recent months, the EU border agency Frontex said Friday, attributing the change to fewer false nationality claims.

Syrians made up 56 percent of all migrants registered in Greece in 2015, but the percentage fell to 51 percent in October, 43 percent in November and 39 percent in December, the Warsaw-based agency said in a statement.