"From this incident, it is clear that we must do more to further strengthen our borders at critical points," von der Leyen wrote in a letter to Spanish Prime Minister Pedro Sanchez seen by AFP.

"The protection of all our external borders is a shared European responsibility," she warned, calling for "vigilant monitoring and the use of physical barriers where needed."

Von der Leyen commended Sanchez on the "swift handling" of the crisis that saw tens of thousands of people stream into Ceuta from Morocco -- almost all of whom have since left.

But the unprecedented scenes in Ceuta triggered sharp tensions among the European Union's 27 member states, with migration hawks Italy and Denmark in particular calling for Spain to be suspended from the Schengen free-travel area.

Madrid pushed back firmly at what it called a "selfish" proposal -- which in any case is legally impossible under EU rules -- while calling for emergency talks among EU interior ministers to chart a way forward.

The videoconference set for Tuesday will be a chance to draw "lessons" from the Ceuta episode, von der Leyen said, warning the challenges of illegal immigration require "a common European response, united action and solidarity".

The commission chief urged the bloc to "double down" in five areas: preventing irregular migration in the first place via cooperation with partner states; strengthening external borders; implementing early warning systems; dismantling smuggling networks and reinforcing returns.

Framing Tuesday's meeting as a "first step," von der Leyen said she expected leaders would take up the issue at their next summit, currently planned for October.

The EU has significantly tightened its migration rules in recent months, including by allowing member states to send failed asylum seekers to so-called "return hubs" located outside the bloc's borders.

Spanish sovereignty 'unquestionable'

Spain insisted Monday its sovereignty over Ceuta was "unquestionable", after tens of thousands of migrants irregularly entered the north African territory from Morocco.

Madrid has avoided criticising Rabat over last week's mass arrivals, which some commentators have interpreted as a means for Morocco to pressure Spain.

Morocco does not recognise Ceuta and Madrid's other north African exclave, Melilla, as Spanish territory and has claimed sovereignty over them since its independence in 1956.

Answering a question during televised remarks on Monday, Spain's Foreign Minister Jose Manuel Albares said: "That Ceuta and Melilla are an integral part of Spain's territory is unquestionable, for the whole world."

Most of the estimated 60,000 people who last week rushed into Ceuta -- most having swum by sea around the border barrier -- have since returned to Morocco.

The Spanish central government's delegate in Ceuta, Miguel Angel Perez, said Monday that 2,500 of them were estimated to still be in Ceuta.

Ceuta's top official Juan Jesus Vivas put the figure at between 3,000 and 5,000.

AFP saw Spanish police and soldiers on Sunday patrolling and escorting small groups of migrants to the border to return to Morocco.

The charity Save the Children said in a statement on Monday that "about 1,000" of the remaining migrants from last week's arrivals were minors who were "unprotected".

It said Ceuta's child protection system had capacity to look after just 100.

AFP on Saturday saw hundreds of children sheltering in a warehouse complex near the border, who said they had no food.

"There was no report alerting us to the size of the numbers that we had to respond to," Spain's Migration Minister Elma Saiz said on Monday in televised comments.

"Of course, this response would not have been possible without Morocco's collaboration."

Some analysts suggested Morocco might have allowed migrants to cross the border to put diplomatic pressure on Spain, which has recently improved ties with Rabat's arch-enemy Algeria.

Spanish officials accused criminal gangs of spreading rumours that a recent Spanish Supreme Court ruling on immigration would allow migrants easier passage.

EU interior ministers scheduled a video call for Tuesday about the Ceuta migrant surge.