The confrontation between Spain and Morocco over the islet of Perejil entered a new and dangerous phase yesterday after elite Spanish assault troops retook the barren rock, capturing six Moroccan soldiers who were immediately returned to their country.

The dawn assault was carried out by special forces, who stormed the tiny island without firing a shot. The Spanish defence minister, Federico Trillo, said his government had accepted, before ordering the operation, that both sides might suffer casualties.

Last night Morocco said the Spanish assault amounted to "a declaration of war". A spokesman for King Mohammed VI accused Spain of wanting to "turn a political problem into a military conflict". Morocco demanded an immediate withdrawal of Spanish troops.

Three Cougar helicopters had swept across the Straits of Gibraltar as dawn broke and deposited 28 soldiers from Spain's special operations command on the highest point of the islet where Moroccan frontier police had raised their country's flag last Thursday.

From there, they moved down the rocky slopes of the islet and, using megaphones, persuaded the six Moroccan soldiers who had replaced the frontier police to give up their arms. Two helicopter gunships circled overhead.

The disarmed Moroccans were taken in launches to the nearby Spanish enclave of Ceuta where, looking dejected, they were sent across the land border into Morocco yesterday morning.

The assault troops left Perejil - Spanish for parsley - a few hours later and were replaced by members of a special operations unit of the Spanish Legion. Mr Trillo said they would remain there, within 200 yards of the coastline of Morocco, until "the situation of the island of Perejil is returned to that previous to July 11".

From the hills beside Bel Younech, a Moroccan coastal hamlet close to Perejil, two red and gold Spanish flags could be seen flying where, until yesterday, the red and green of Morocco had been on display. It was the first time a Spanish flag had been seen on the island since it was last used as a military observation post in the early 1960s.

Mr Trillo, visibly pleased at Spain's first military victory for several decades, described the surprise assault on the islet as "clean" and said the Spanish soldiers had orders to shoot only in self-defence.

He said the operation had been an act of legitimate defence. "Spain had been attacked by the force in a sensitive point of its geography," he said.

The foreign minister, Ana Palacio, told an emergency session of the Spanish parliament the assault was not an attempt "to impose any solution by force" but to restore the uninhabited islet to the previous status quo. "Spain has no interest in maintaining a permanent military presence," she said.

But Morocco, which last week feigned surprise at Spanish anger over its decision to occupy the islet, demanded the Spaniards abandon it immediately.

"The kingdom of Morocco protests with force against this unjustified aggression at the moment when Morocco and Spain were trying to resolve this crisis by diplomatic means," an official statement said.

"The Moroccan government urges the immediate and unconditional withdrawal of Spanish forces. The island is an integral part of Moroccan territory."

In the northern provincial capital of Tetouan, about 25 miles from the islet that Moroccans call Leila, about 200 people demonstrated in front of the Spanish consulate. "Out of Leila, out of Ceuta, out of Melilla," they shouted, referring to Spain's other north African enclave further east, near the Algerian border.

Nato officials said yesterday the alliance was "pleased the status quo ante has been restored". Morocco said it would take its protest to the UN.