World leaders have responded with shock, concern and solidarity after several deadly explosions hit Brussels in a suspected terrorist attack on European soil. At least 13 people are reported to have been killed by two blasts that ripped through the departure hall of Zaventem airport in the Belgian capital. All flights have been cancelled and inbound planes diverted elsewhere.

Explosions were also heard later on the city's Metro system. The terror alert in the northern-European country has been raised to maximum.

Belgian Prime Minister Charles Michel wrote:

I strongly condemn these hateful attacks. Our thoughts go out to the victims and their families. We stand united against terrorism. — Charles Michel (@CharlesMichel) March 22, 2016

British Prime Minister David Cameron tweeted:

PM: I am shocked and concerned by the events in Brussels. We will do everything we can to help. — UK Prime Minister (@Number10gov) March 22, 2016

He was echoed by his Italian counterpart, Matteo Renzi, who wrote (translated): "with heart and mind to Brussels, Europe."

Con il cuore e con la mente a Bruxelles, Europa — Matteo Renzi (@matteorenzi) March 22, 2016

Ukrainian president, Petro Poroshenko, sent his condolences to the families of the victims.

Shocked by deadly terrorist act in heart of united Europe. Condolences to Families of victims. We must stop terrorism together — ÐÐµÑÑÐ¾ ÐÐ¾ÑÐ¾ÑÐµÐ½ÐºÐ¾ (@poroshenko) March 22, 2016

British foreign secretary Philip Hammond described the attacks as an act of "senseless violence".

Deplore senseless violence of #Brussels attack. UK offering #Belgian authorities every assistance we can. We stand united against terrorism — Philip Hammond (@PHammondMP) March 22, 2016

Russian Prime Minister Dimitry Medvedev said the blasts were "a barbaric crime against civilians".

The attacks in Brussels were a barbaric crime against civilians. We offer our condolences to the victims' families https://t.co/xLa4PNyiA2 — Dmitry Medvedev (@MedvedevRussiaE) March 22, 2016

Lithuania's president Dalia Grybauskaitė, described the blasts as a "terrible act of terrorism", while former labour leader Ed Miliband said he was "shocked and saddened" by the news.

Terrible acts of terrorism in #Brussels. Stand in solidarity with all those affected. — Dalia GrybauskaitÄ (@Grybauskaite_LT) March 22, 2016

Shocked and saddened to hear about terrible events in Brussels. My heart goes out to those affected. — Ed Miliband (@Ed_Miliband) March 22, 2016

French president Francois Hollande said: "I express my full solidarity to the Belgian people. It is the whole Europe that has been hit by the attacks in Brussels."

Jâexprime mon entiÃ¨re solidaritÃ© avec le peuple belge. Ã travers les attentats de Bruxelles, câest toute lâEurope qui est frappÃ©e. — FranÃ§ois Hollande (@fhollande) March 22, 2016

Former French PMs François Fillon and Alain Juppé sent messages of solidarity for their Belgian neighbours.

"Solidarity with Brussels. Radical terrorism is a cancer. Europe must stand togheter," tweeted Fillon (translated).

"Solidarity with the Belgian people after the explosions in Brussels and at Zaventem airport. Support to rescuers and injured," wrote Juppe (also translated).

FraternitÃ© avec #Bruxelles ! Le terrorisme intÃ©griste est un cancer. L'Europe doit faire bloc ! — FranÃ§ois Fillon (@FrancoisFillon) March 22, 2016

SolidaritÃ© avec le peuple belge aprÃ¨s les explosions Ã #Bruxelles et Ã l'aÃ©roport de #Zaventem. Soutiens aux blessÃ©s et aux secours. ð«ð·ð§ðª — Alain JuppÃ© (@alainjuppe) March 22, 2016

The President of the European council Donald Tusk said the EU will help Belgian authorities as well as other European government to counter the terror threat. "I am appalled by the bombings this morning at Zaventem airport and the European district in Brussels which have cost several innocent lives and injured many others," he wrote in a statement. "I extend my sincerest sympathies to the relatives and friends of the victims. These attacks mark another low by the terrorists in the service of hatred and violence."