Serbia will form joint army and police patrols to stem the flow of refugees and migrants, Prime Minister Aleksandar Vucic has said.

Speaking on Saturday, Vucic said Serbia cannot become a "parking lot" for refugees and migrants heading to Western Europe.

The number of refugees and migrants has grown since Hungary introduced harsh new border controls, leaving thousands bottlenecked on the Serbian side of the border in recent weeks.

"We will form joint army and police teams to protect our border," Vucic told reporters, but failed to mention how many troops would be involved.

An estimated 2,669 refugees and migrants - including Afghans, Pakistanis and Syrians - are currently in Serbia, Vucic said.

Vucic threatened that refugees and migrants entering Serbia without documents - and who haven't applied for asylum - will be expelled from the country within 30 days, adding that the country can accommodate between 6,000 and 7,000 asylum seekers.

More than a million refugees and migrants reached European shores by boat in 2015, according to the United Nations refugee agency, the UNHCR.

READ MORE: Refugees in Bulgaria endure 'extortion, robbery, violence'

Nearly 240,000 people have made the journey across the Mediterranean so far this year.

Serbia lies on the Balkan route crossed by hundreds of thousands of people fleeing war and economic devastation since last year on their way to Western Europe.

Although the route was effectively closed in March when the European Union and Turkey reached a deal to halt the flow of refugees to Europe, many people continue to make the treacherous journey, usually with the help of human smugglers.

Serbian authorities said recently that 102,000 refugees and migrants had been registered since the start of the year - more than 500 a day.

'Push backs'

The UNHCR has expressed concern over allegations that Hungary has been using "push backs" - a practice barred under international law - to keep refugees and migrants from crossing into Hungary from Serbia.

An estimated 1,400 refugees and migrants are stuck on the Serbian side of the border due to new Hungarian measures.

In September 2015, Hungary built a razor-wire fence along the 175-kilometre border in order to stem the flow of refugees and migrants.

"We are deeply concerned about further restrictions by Hungary leading to push-backs of people seeking asylum and reports about the use of violence and abuse," UNHCR spokesman William Spindler said in a statement.

READ MORE: Hungary - three villages and the fence that divides them

Introduced last week, Hungarian forces can detain and expel any refugee or migrant found within eight kilometres of the border without a legal process.

Human Rights Watch (HRW) has condemned the measures, saying they violate international law.

In a report released earlier this week, HRW detailed allegations that Hungarian forces have used violence and force against refugees and migrants.

The report quoted an unidentified man as saying: "I haven't even seen such beating in the movies."

"Five or six soldiers took us one by one to beat us. They tied our hands with plastic handcuffs on our backs. They beat us with everything, with fists, kicks and batons. They deliberately gave us bad injuries."

Lydia Gall, HRW's regional researcher, said that "the abuse of asylum seekers and migrants runs counter to Hungary's obligations under European Union law, refugee law and human rights law."