The European Union flag is seen flying, at the border of Gibraltar with Spain, in the British overseas territory of Gibraltar, historically claimed by Spain, June 27, 2016, after Britain voted to leave the European Union in the EU Brexit referendum. REUTERS/Jon Nazca

BRUSSELS (Reuters) - Leaders of the European Union institutions invited Donald Trump to a summit as soon as he can schedule one and cited security and trade ties as key in a letter on Wednesday congratulating him on his election as U.S. president.

“Today, it is more important than ever to strengthen transatlantic relations,” wrote Donald Tusk and Jean-Claude Juncker, presidents respectively of the European Council, which groups member states, and the executive European Commission.

“Only by cooperating closely can the EU and the U.S. continue to make a difference when dealing with unprecedented challenges such as Da’esh (Islamic State), the threats to Ukraine’s sovereignty and territorial integrity, climate change and migration.”

“Fortunately, the EU-U.S. strategic partnership is broad and deep: from our joint efforts to enhance energy security and address climate change, through EU-U.S. collaboration on facing threats to security in Europe’s Eastern and Southern neighborhoods and to the negotiations on the Transatlantic Trade and Investment Partnership (TTIP) - we should spare no effort to ensure that the ties that bind us remain strong and durable.”