France is to send an extra 1,600 police to public transport hubs following the blasts at Brussels Zaventem airport and at the Maalbeck metro station, Interior Minister Bernard Cazeneuve announced after an emergency meeting with other ministers on Tuesday morning. Flights from France and Belgium and direct trains between Paris and Brussels have been stopped.
The 1,600 police and gendarmes were to be sent to airports, rail and metro stations and the borders, Cazeneuve said.
Some 400 of them will be in the Paris regions, notably at Charles De Gaulle and Orly airports.
"We are at war," Prime Minister Manuel Valls said. "We have been subjected to acts of war in recent times in Europe."
French President François Hollande expressed his solidarity with the Belgian people in a statement.
Thalys trains between France and Belgium have stopped at the Belgian border and public transport in Brussels was closed following the attacks.
The death toll was at least 26, 11 at the airport and 15 at Maalbeck metro, with 55 injured, the Belgian emergency services said late on Tuesday morning.
Public prosecutors said 13 people died and 35 were wounded at the airport, where police say the blasts were a suicide attack.
They came as Paris attacks suspect Salah Abdeslam was being interrogated, following his arrest in Brussels on Friday.
Investigators said he had been planning further attacks and was working with a number of accomplices.
Zaventem airport was closed and five international flights supposed to go there were redirected to French airports. One, coming from Rome, landed at Lille, one, from Barcelona, at Paris Orly and two, from Tirana and Montreal, at Paris Charles De Gaulle.
All flights from France to Belgium were cancelled.
Belgian Prime Minister Charles Michel condemned a "blind, violent and cowardly" acts at a press conference.
European Council President Donald Tusk was "appalled" by the attacks.
"The European institutions are hosted in Brussels thanks to the generosity of Belgium's government and its people," he said in a statement. "The European Union returns this solidarity now and will fulfill its role to help Brussels, Belgium and Europe as a whole counter the terror threat which we are still facing."