An estimated 1,000 refugees and migrants are stuck on the Greek border with Macedonia after some were refused entry because of their nationality, the United Nations said.
"A new humanitarian situation is developing in Europe that needs urgent attention," Adrian Edwards of the UN High Commissioner for Refugees (UNHCR) told a news briefing.
Six Iranian refugees have sewn their mouths shut as part of a protest at the Greece-Macedonia border.
They took the step as hundreds of refugees, many from Iran and Morocco, protested for a fourth successive day at the border near the village of Idomeni.
Macedonia and other Balkan countries have tightened border controls in the wake of the deadly Paris attacks on November 13, which left 130 people dead.
Police said 2,900 people crossed the border into Macedonia in the 24 hours before 6am local time on Monday, down from more than 6,000 the previous day.
A bomb exploded outside offices in central Athens early on Tuesday, smashing windows but causing no injuries.
Police believe the blast, outside a Greek business federation, was carried out by domestic guerrilla groups.
Attacks against banks, politicians, journalists and businesspeople are not uncommon in Greece, although Tuesday's blast was the first such attack since Prime Minister Alexis Tsipras came to power in January.
The device, triggered by a timer and placed in a backpack near the entrance of the Hellenic Business Federation offices, went off at around 3.30am local time (1.30am GMT).
Police cordoned off a two-block area surrounding the building and bomb disposal squads examined the scene.
At least 22 refugees have died after two boats travelling from Turkey to Greece sank overnight, authorities say.Read the full story ›
ITV News understands seven people have been found dead after Wednesday's sinking off Lesbos, with around 38 still missing.Read the full story ›
At least three people have drowned and over 240 were rescued when a wooden boat carrying migrants sank off the coast of Lesbos.Read the full story ›
A notorious human trafficker shown by ITV News at Ten smuggling refugees from Turkey to Greece is arrested after being caught on camera.Read the full story ›
The minister responsible for Syrian refugees in the UK has refused to reveal how many people have arrived in Britain under the government's resettlement programme.
Richard Harrington said he was not prepared to provide a "running commentary" on the numbers brought to the UK since David Cameron announced plans to accept 20,000 Syrian refugees over five years.
He told MPs at the home affairs select committee that he knew how many people had arrived since the initiative was launched last month but repeatedly declined to give figures.
Mr Harrington, who was appointed to the newly created ministerial post last month, said: "I don't think anything will be helped by my giving a running commentary on numbers."
His refusal to disclose the information drew criticism from the committee chairman, Keith Vaz.
We will have to write to the Prime Minister now and tell him that we need this figure because frankly I think it is unsatisfactory for you to come before this committee and not have the numbers. The public is very interested in this.
An 11-month-old boy who drowned when a dinghy carrying more than 60 refugees was swamped by waves during a night-time crossing to the Greek island of Lesbos has been buried.
Omaar, from Idlib, Syria, drowned when waves swamped the dinghy last Thursday.
Coastguards worked to save the baby but were unsuccessful.
A funeral service was held for the baby in Lesbos today.