By Brenna Hughes Neghaiwi

ZURICH Aug 10 (Reuters) - Amnesty International warned of a buildup of migrants on Italy's border with Switzerland and demanded clarification from Swiss authorities over reports by children that they had been sent back when trying to join their parents there.

Switzerland said the buildup was due to an influx of African migrants seeking passage to north European countries such as Germany. Any individual requesting asylum would be granted the opportunity.

Several hundred migrants have been sleeping near the train station in Como, Italy, since July after a Swiss clampdown on crossings.

"We're concerned about reports from minors who by their own accounts were sent back to Italy at the Swiss border and were prevented from joining family members in Switzerland," Amnesty International Switzerland said in a statement on Tuesday.

"If a minor has family members in Switzerland who could care for her or him, ultimately Switzerland should process that asylum request," the agency added.

Some two-thirds of the nearly 7,500 migrants who reached Switzerland via the southern canton of Ticino have been turned back since early July, a steep rise from the one in seven denied entry earlier this year.

That proportion was still rising in recent weeks.

Swiss authorities said this was due to an influx of people -- mainly from Eritrea, Gambia and Ethiopia -- wishing to transit Switzerland from Italy to Germany or other northern European countries, which requires a valid permit.

But any individual requesting asylum in Switzerland -- or communicating a desire to do so to border guards -- would be granted the opportunity, customs and migration authorities said.

That practice hadn't changed in recent weeks, they said.

Martin Reichlin of the State Secretariat for Migration (SEM) said he would expect any child arriving at the border and attempting to join relatives in Switzerland to be delivered to the care of his organisation.

Authorities have a responsibility to inform minors of their rights, Amnesty said, and a systematic return of children would be incompatible with the U.N Convention on the Rights of the Child.

"Recognising the precarious circumstances for refugees in northern Italy, it's unacceptable to turn away especially vulnerable people," Amnesty said.

Migrants turned back at the French and Swiss borders are beginning to pile up in Milan, the city's mayor, Giuseppe Sala, said on Tuesday. More than 3,000 migrants in transit to other European countries were stranded in Italy's financial capital. (Reporting by Brenna Hughes Neghaiwi in Zurich and Steven Scherer in Rome; editing by Ralph Boulton)