By Tom Esslemont

LONDON, April 16 (Thomson Reuters Foundation) - Many migrants who survive the perilous sea crossing to Europe from North Africa are physically and sexually abused by the smugglers who organise their journey, according to testimony gathered by aid workers in Italy.

"We can clearly say that violence is one of the methods used to control migrants," said Stefano Di Carlo, Head of Mission for Medecins sans Frontieres (MSF), the medical aid agency.

His observations show there is a pattern in which traffickers are using increasingly audacious tactics to carry out the dangerous crossing.

In December and January successive groups of migrants, many from Syria or the Horn of Africa, were stranded aboard large vessels placed on a collision course with the Italian coast, having been abandoned by their crew.

About 400 migrants are believed to have died when their crowded boat capsized on the way to Italy from Libya on Tuesday.

"Clearly the safety of the migrants is not the traffickers' top priority," said Di Carlo, whose teams assist migrants arriving along the southern coast of Sicily.

Migrants report having to pay huge sums before leaving Libya for a place on a boat, with estimates ranging from $400 to $2500 per person.

"Some migrants, who've fled persecution in their countries of origin, change their minds when they get to the beach, so smugglers force them to get on board, with knives, sticks and guns," said Flavio Di Giacomo, spokesman for the International Organisation for Migration (IOM).

Anyone who refuses to go risks imprisonment, Di Giacomo says.

One young Somali woman, having crossed the Libyan desert to reach the North African coast, told MSF she was taken hostage when she arrived. While waiting for her family to send a $1,500 ransom fee, she was beaten, she said.

"Other women were raped," she told MSF on arrival in Italy.

"I was not raped because I speak Arabic [the language of her captors]."

In Libya, a fragile government has struggled to maintain law and order as rival militias fight for control.

"Civil conflict creates the conditions for aggravated smuggling to thrive," says Christopher Horwood, Coordinator, Regional Mixed Migration Secretariat (RMMS) in Nairobi.

In a statement issued on Wednesday, Yury Fedotov, the director of the U.N. Office on Drugs and Crime (UNODC), said "migrant smuggling exploits desperation and provides the criminal networks with enormous profits."

The organisation aims to train frontline anti-trafficking officers, but a UNODC spokesman said its operations in Libya were discontinued in 2014 because of the rapid deterioration of the political and security situation.

(Reporting By Tom Esslemont; Editing by Tim Pearce)