The UN’s refugee agency has warned that the number of refugees and migrants entering Europe, which currently stands at 8,000 a day, will be “the tip of the iceberg” unless there is an end to the civil war in Syria.

Amin Awad, regional refugee coordinator for the UN high commissioner for refugees (UNHCR), said Europe needed to prepare for millions more migrants as well as work to bring an end to the Syrian conflict to prevent a bigger movement of people.

“I don’t see it abating, I don’t see it stopping. If anything it gives an indication perhaps that this is the tip of the iceberg,” Awad told Reuters.

“As long as there is no solution for Syria, as long as there is no stabilisation of the refugee conditions in the neighbouring countries … so efforts have to be made.”

Asked if Europe should prepare for millions more migrants, he said: “If it happens that in a few months you have a over half a million, it could certainly happen.”

Countries needed to unite behind a global approach, he said, suggesting that the onus was on the five permanent members of the UN security council to stop the war.

“The governments that could make a difference are the governments that are responsible for influencing the global political and security fate of our world today. The leaders of our generation have to move very quickly to find a resolution to the Syria problem before it becomes a global one.”

Awad welcomed the EU’s decision to contribute €1bn (£740m) to the UN humanitarian effort but said even that was not enough, and the situation needed more than cash.

The UN is also bracing for a worsening of the humanitarian situation in Syria’s neighbour Iraq.

Dominik Bartsch, the UN’s deputy humanitarian coordinator in the country, said 10 million people in Iraq were expected to need humanitarian support by the end of the year, with 3.2 million already displaced.

He said the UN was planning for the displacement of 500,000 people from the Iraqi city of Mosul if Iraqi forces launch an attempt to recapture the city from Islamic State.

Croatia said on Friday that it might lift a blockade on its border with Serbia in the next 24 hours. The two Balkan neighbours have been engaged in a trade war this week over the flow of thousands of migrants across their joint border. Zagreb banned Serbian vehicles on Thursday, in response to Serbia blocking Croatian goods and trucks.

“I am having intense discussions with my colleagues in the government to lift the border measures today or tomorrow,” the Croatian prime minister, Zoran Milanović, said on the government’s official Twitter account.

Croatia’s move comes as the commissioner for European neighbourhood policy and enlargement negotiations tours a Serbian migrant camp near the border. Johannes Hahn, arrived at the border zone on Friday, together with Serbia’s prime minister, Aleksandar Vučić. Ahead of his visit, Hahn praised “Serbia’s competent and resilient management of the refugee crisis”, adding: “The EU stands by Serbia in this pan-European crisis which can be solved only with a common approach.”

On Friday, Slovenia’s foreign minister said his government was not informed in advance of Hungary’s decision to build a razor-wire fence on the border between the two EU countries, and that the fence was “not necessary”. Both nations are part of the EU’s passport-free Schengen travel zone where people are supposed to be able to cross borders freely.

Hungarian government spokesman Zoltán Kovács said on Friday that the Slovenian border was meant to “block direct detours” by migrants trying to avoid fences on Hungary’s borders with Serbia and Croatia. Hungary is the most direct way to reach western Europe.