The confirmed death toll from the two explosions at around 8:20 am at Brussels airport had reached 14 by mid-afternoon on Tuesday, The Local France reports, while dozens more people were injured.
At least twenty people people were also killed in a separate bomb attack on the metro system in central Brussels at around 9:20 am, with as many as 55 injured according to Belgium's minister of health.
Belgian prosecutors confirmed that the explosions at the airport were suicide bombings.
The blasts came just days after the arrest of Salah Abdeslam, the chief suspect in the terrorist attacks that claimed 130 lives in French capital Paris in November.
In a press conference at midday on Tuesday, Interior Minister Thomas de Maiziere said that there was so far no indication German citizens were involved in the attacks, either as victims or perpetrators.
Germany's federal police "has intensified its protective measures on the borders, including on the borders of the Benelux [Belgium, the Netherlands and Luxembourg] states and around critical infrastructure such as train stations and air transport," a statement from the force's headquarters in Potsdam read.
Officers would be warned to watch out for attacks directed at them and equipped with additional protective gear, the statement continued.
German airports on high alert
Officers at Frankfurt airport were immediately placed on high alert and additional patrols were ordered for sensitive areas, broadcaster n-tv reported early on Tuesday.
Many flights heading to Brussels are likely to be rerouted to Frankfurt after the Belgian capital's airport was closed completely, Frankfurter Rundschau reported.
A federal police spokeswoman told The Local that they were still evaluating the situation following the explosions in Brussels and would give details of security measures at other airports as soon as possible.
One The Local reader reported increased police presence at Cologne airport on Tuesday morning.
Berlin's Tegel airport announced that its observation platform would remain closed on Tuesday as a "preventive measure".
Two flights to Brussels from Tegel slated for Tuesday were cancelled, including one aircraft whose passengers had boarded and were ready to take off.
There had been a total of eight flights from Berlin to Brussels from both Tegel and Schönefeld airport, but with Brussels airport completely closed all have been cancelled.
Es wurden alle Flüge von und zum Flughafen Brüssel für heute gestrichen. Kontaktiert bitte eure Airline. https://t.co/mjomIpkgoY— Berlin Airport (@berlinairport) 22. März 2016
Lufthansa, Europe's largest airline, has cancelled all of its flights to Brussels on Tuesday. Flights that were already in the air have been diverted, a spokesman told Tagesspiegel.
Border checks stepped up
A federal police spokesman said that checks have been stepped up and more patrols are on duty on the short German-Belgian border in Rhineland-Palatinate since Tuesday morning, broadcaster SWR reported.
"The police has intensified its investigative activities in the border zone with the involvement of special response units," Rhineland-Palatinate Interior Minister Roger Lewentz said.
"We will do everything in Rhineland-Palatinate to ensure security for our people," state minister-president Malu Dreyer said, adding that people in the western state have close contact with their Belgian neighbours and would be keeping them in their thoughts.
Metro stations also attacked
Federal police were also unable to comment on whether security would be increased on local transport services in German cities after explosions were also reported on the Brussels metro system.
Just arrived at Schuman station after walking on the tracks. pic.twitter.com/4xc0YCQmIv— Evan Lamos (@evanlamos) 22. März 2016
Witnesses spoke of panic in the streets as survivors stumbled out of Maalbeek Metro station covered in blood and suffering burns.
Area around Maelbeek metro sealed off, I can see 4 ambulances, 1 fire engine. pic.twitter.com/xFmMBqcVdZ— Jennifer Rankin (@JenniferMerode) March 22, 2016
Germany's national rail operator Deutsche Bahn said that all trains from Germany to Brussels were cancelled, effective immediately.
"We are taking all customers' tickets back until the end of Wednesday," a spokesperson told the Rheinische Post.
Two explosions in airport
Brussels airport confirmed that there had been two explosions shortly after 8am and warned passengers not to come to the airport, which would be closed until further notice.
Belgian police confirmed that at least one person died and several were injured in the blasts.
However other unconfirmed reports say as many as 11 could have been killed. One witness said children were among the injured.
Bomb explosion at Brussel airport. Apparent terrorist activity. Airport is on lockdown and being evacuated. pic.twitter.com/nPRbdzFQdO— Taa Wongbe (@twongbe) 22. März 2016
Belgian authorities have implemented their emergency response plan and the terror alert has been raised to its maximum level.
The city has since gone into lock-down with the capital's Metro system closed down and unconfirmed reports the main train station has been evacuated.
A section of the airport's roof is believed to have collapsed amid the explosions, one of which is believed to have occurred in the departure lounge.
Images and videos showed smoke rising above the airport while passengers fled in panic. Other images showed injured passengers lying on the ground, some covered in blood, with their clothes ripped off in the explosion.
The cause of the explosions is unknown, but they come at a time when the Belgium capital is on high alert for terror attacks after the arrest of Salah Abdeslam, the chief suspect in the Paris November attacks.
One witness named David Crunelle told L'Express newspaper: "I heard two distinct explosions about three seconds apart. They occurred near the international check-in desks for flights to the United States.
"After the explosion, everything became dark. There was a lot of smoke but no flames. There was no movement of panic but people began to get out of the airport straight away.
"I saw at least six people severely injured, including children. There are a lot of people lightly injured, covered in blood. Many are crying," he said.
Many ambulances arrive at the Airport. Passengers from Terminal A waiting outside, Police checks building. pic.twitter.com/DL3YoGFV0M— Katrin Kuntz (@katrinkuntz) March 22, 2016
Germans warned to stay vigilant in Brussels
"The highest terror alert level of four applies across the whole country [of Belgium]. Count on delays," the German Foreign Ministry said in updated travel advice for Belgium on its website.
"Travellers in Brussels are urgently requested to only move around the city with increased vigilance and attention, to avoid large gatherings of people and to follow the instructions of Belgian security services without fail."