Secretary of State John F. Kerry, left, and Greek Foreign Minister Nikos Kotzias finish a news conference Friday at the Ministry of Foreign Affairs in Athens. (Jonathan Ernst/Reuters)

Secretary of State John F. Kerry said Friday that Syrian opposition groups will need to be assured, before they join peace talks and the fight against Islamic State militants, that President Bashar al-Assad will eventually leave power.

“It will be exceedingly difficult to cooperate without some indication or confidence that in fact there is a resolution in sight,” Kerry said in a news conference in the Greek capital, adding that the lack of guarantees would be interpreted as helping to entrench Assad’s power.

Kerry is among the diplomats from more than a dozen countries that have interests in resolving the Syrian conflict and are trying to arrange negotiations to end almost five years of war. Russia and Iran continue to back Assad, however, and it is uncertain whether it will be possible to start direct negotiations between the government and the rebels by early next year as the diplomats hope.

“It’s not clear he would have to, quote, go, if there was clarity in respect to what his future might be,” Kerry said, adding that by clarity he meant Assad’s eventual departure.

Kerry was in Greece on the final leg of a week-long trip to Europe, where he held a series of talks centered on the war in Syria and efforts to combat Islamic State militants in their strongholds there.

What started out as simple protests in Syria has expanded to civil war and now an international crisis.

His stop brought him to a country bent under the burden of caring for more than 700,000 refugees and migrants, many of them Syrians, who have landed in Greece this year by sea from Turkey. The European Union has been pressuring Greece to control its borders better and register all refugees properly, even as it enacts austerity measures related to its economic crisis.

The day before Kerry arrived, the Greek coast guard recovered the body of a woman who drowned when the small boat she was in, also carrying other migrants, capsized in the Aegean Sea. She was the latest of hundreds of refugees and migrants who have drowned trying to reach Greece in small, decrepit boats operated by smugglers who charge exorbitant fees.

After meeting with Prime Minister Alexis Tsipras and Foreign Minister Nikos Kotzias, who characterized Greece’s situation as being in the middle of a “triangle of instability,” Kerry made note of Greece’s travails.

“I want to commend the resilience and warmth of the Greek people,” Kerry told reporters. “Even as you work your way out of one of the worst economic crises in your history, Greece has opened its doors to refugees and migrants escaping the fighting in Syria and neighboring countries.”

Calling Greece a front-line state, Kerry announced that the United States would donate almost $24 million to the U.N. High Commissioner for Refugees and urged other countries to make more contributions to the cash-strapped agency.

The UNHCR will spend the money on food, water and temporary shelter for refugees, and to establish facilities to screen and process refugees, he said. It was not clear how much would go to aid refugees in Greece, as some of it will support the UNHCR’s work in the Balkans and elsewhere in Europe. Since the crisis began, the United States has donated more than $4.5 billion in humanitarian aid for refugees and to help the countries that are hosting them.

“This is not solely a Greek crisis. This is not solely a European crisis or even a Middle East crisis,” Kerry said. “It is a global crisis for which we must all share responsibility.”