(Reuters) - The Slovak opposition SMER-SSD party, which leads polls ahead of an election this month, called on the government on Wednesday to start border checks to halt an increasing flow of migrants crossing from Hungary to Western Europe.

Slovakia has seen a jump in the number of migrants, many from Afghanistan and Syria, crossing in recent months, and has been letting them through after registration.

Party head Robert Fico, a former prime minister, has been running on a campaign to defend national interests, end military support for Ukraine and oppose any sanctions on Russia that could hurt Slovakia.

"We want to remind the Slovak government that it has all options - legislative, technical and personnel - to revive border controls on the Slovak-Hungarian border," Fico told a news conference shown live on Facebook.

"This is a fundamental step we have to make. Unless we start halting the flow of illegal migrants to Slovak territory, we will not be able to resolve the situation inside Slovakia."

The caretaker government has said it is impossible to seal the border with Hungary, which stretches over 650 km (400 miles).

It has sent up to 500 soldiers to help police patrolling border areas and taken measures to register migrants quickly.

Police have said migrants are lured to Slovakia because, under a law approved under Fico's government, it is registering Syrians and Afghans and giving them the right to stay.

The Interior Ministry said last week the number of detained illegal migrants had soared ninefold from a year ago, to more than 27,000 so far this year.

Slovakia holds an early election on Sept. 30, after a centre-right coalition collapsed last year and a caretaker cabinet took office in May.

SMER-SSD has led opinion polls with around 20% backing. The party is a member of the left-leaning S&D group in the European Parliament but has taken increasingly socially conservative and anti-Brussels positions.