“Let’s not fool ourselves. From the point of view of some of our traditional partners - and I am thinking here about transatlantic relations - there is no eternal guarantee of close cooperation with us Europeans”, she said in a speech in Belgium.

She said Europe was facing one of its “biggest challenges for decades” due to conflicts on its borders, such as Russia’s aggression in Ukraine.

She added that it would be "naive always to rely on others who would solve the problems in our neighbourhood”.

She also said that the UK’s decision to leave the EU ought to galvanise military cooperation among the 27 remaining member states.

“We should see this [UK] decision as an incentive to work together, to hold Europe together now more than ever”, she said.

Merkel was in Belgium to receive degrees from Leuven and Ghent universities and to meet the Belgian royal family.

Her appeal for the EU to step up security cooperation comes after the election of Donald Trump in the US, who has said he might not defend Nato allies and that he might accept Russia’s annexation of Crimea from Ukraine.

The US is currently deploying tanks and soldiers in Poland and in the Baltic states as part of wider Nato plans to deter Russian aggression.

The pre-Trump US administration reportedly sped up the deployment in case he changed his mind, but Trump’s new security chiefs have spoken out on Russia in surprisingly strident terms.

Mike Pompeo, Trump’s nominee for CIA chief, said on Thursday in a senate hearing that Russia was "asserting itself aggressively" in Europe.

James Mattis, the defence secretary nominee, said Russia was “trying to break the North Atlantic alliance [Nato]”. He said Nato was “under the biggest attack since World War II, and that's from Russia”.

Security crisis

Merkel, earlier on Thursday, also met with Luxembourg prime minister Xavier Bettel and with the Grand Duchy’s royal family.

She told press in Luxembourg that unless the EU improved security in the passport-free Schengen travel zone and established control of its external borders, then it risked collapse.

She said that the case of Anis Amri, the man suspected of the Christmas terrorist attack in Berlin, who moved freely across EU borders, proved the “urgency” of the issue.

“Everyone knows: If we don’t succeed, each country will install its own border controls. Then freedom of movement will no longer be possible, so to speak, not only the free movement of people but also of goods, financial services, and services,” she said.

She added that the EU should make more deals such as the one with Turkey to stop migrants from coming.

The EU is paying Turkey to take care of refugees on its territory.

The set-up has been criticised on human rights grounds, but Merkel said “agreements between states lead to reasonable border control” and that “with north African states and with African states as a whole, as we did with Turkey, we also need agreements”.

She spoke after visiting the home of Robert Schuman, a post-World War II politician who helped to found the EU.

Emotional Brexit

She said the visit was an “emotional moment” for her and that the EU should stick together in the way it handled Britain’s departure - the first ever in EU history.

“We must not be allowed to divide ourselves apart. The 27 have to appear together in the negotiations”, she said.

She added that EU states should consider harmonisation of corporate tax in order to compete with the City of London after it leaves.

“This will become more of an issue if we’re talking about lower tax rates, for example in the UK”, she said.

Bettel said the refugee crisis remained “a major task” for Europe and criticised countries that did not want to share asylum seekers with front-line states.

“When different countries speak of flexible solidarity, that is … problematic for us. We need rules, and when we agree on these rules, each country must respect them”, he said.

Problematic flexibility

Flexible solidarity was a policy endorsed by Slovakia and other central European states to replace a previous EU decision on binding migrant quotas.

It was meant to let states pay the EU to avoid taking in migrants.

Merkel was initially an advocate of an EU open-door policy on refugees, but has become less welcoming to migrants as the elections approach.

A Flemish far-right group, Voorpost, protested against her university awards in Belgium on Thursday, according to the AFP news agency.

About 50 people held banners, including posters that branded her “GUILTY” of the Christmas attack, in which Amri, who is of Tunisian origin, killed 12 people.