ISIS has threatened to hit the UK 'harder and more bitter' after claiming responsibility for the Brussels bombings today.

Britain has ramped up security with armed officers taking to the streets near major landmarks, train stations and airports.

The heightened police presence comes after at least 34 people were killed and many more left injured in a series of bomb attacks inside an airport terminal and Metro station in the Belgian capital.

The explosions come just a day after the Belgium Interior Minister warned of possible revenge attacks after the arrest of Paris massacre suspect Salah Abdeslam in the city last week.

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Pictured are the chaotic scenes at Brussels Airport after two explosions inside the terminal earlier today

Armed British police officers stand on duty outside the entrance to the Houses of Parliament in London today

In London: Armed officers make their way through the Eurostar terminal at St Pancras station. There was an increased security presence at transport hubs across the city

An armed policeman on duty in Downing Street, in London, today following Brussels bomb attacks

Armed police at the entrance to the Eurostar at St Pancras Station in London. Journeys to Brussels were disrupted and terminated in Lille in France after this morning's terrorist attacks

Security levels at British airports, railways stations, at the borders and on city streets have been increased following the explosions in the departure hall of Zaventem Airport and on the metro system in Brussels. Pictured is an armed officer in Central London

Armed police officers in central London after the terrorist attacks in Brussels earlier today

A soldier walks through debris after two explosions rocked a terminal building at Brussels Airport

A person is carried to safety as troops helped the injured and secured the area after the explosions in the terminal building

And through the terrorist group's official Telegram account, a statement read: 'We promise to the states that are allied against the Islamic State that they will face dark days in return for their aggression against our state.

'And what will await you will become harder and more bitter through the grace of Allah.

'In praise of Allah who has made our blows precise and helped us succeed. we demand for him to accept our brothers among the martyrs.'

A major manhunt is underway for an ISIS suspect in a white coat and black hat who fled Brussels Airport after the two explosions ripped through the terminal in a suicide bomb attack this morning, killing 14 people and injuring dozens of others.

Police issued a wanted notice for the man who was seen on CCTV pushing a luggage trolley through the check-in area with two other suspects minutes before the blasts.

His alleged accomplices were wearing black gloves on their left hands, which security sources say would have hidden the triggers for their explosive vests.

The two men blew themselves up while the third suspect is believed to have left a nail bomb and fled, according to De Standaard.

It is not known if the fugitive then sped to Maelbeek station to carry out the blast that killed 20 people in a subway train just 79 minutes later.

First picture: These three men, pretending to be air passengers, are believed to the terrorists who have carried out the Brussels airport. The two suspected suicide bombers on the left were both wearing black gloves - which the Belgian media says would have hidden the triggers for their explosive vests. The third suspect in the hat is believed to still be on the run after dropping his nail bomb

Terrified passengers at Brussels Airport have told how there 'just blood' everywhere and likened scenes after the bomb blast to the 'apocalypse'

Around 90 minutes later, 10 were killed when an explosion hit a Metro station near the EU headquarters in the city centre in another suspected terror attack

Carnage: At least eleven people have died and several injured after two explosions rocked Brussels Airport in a suspected terror attack

Alphonse Youla (pictured), who was working on a stand putting security wrapping around suitcases, said he heard 'a man shout some Arabic words then an explosion'

Alphonse Youla, who was working on a stand at the airport putting security wrapping around suitcases, said: 'I heard a man shout some Arabic words then an explosion.. then a second explosion, a massive explosion, much bigger.

'It was a horror. I saw at least seven people dead. There was blood. People had lost legs. You could see their bodies but no legs.'

Belgian Prosecutor Eric van der Sypt confirmed two bombers died at Brussels airport and said a third is being 'actively' sought. Raids are currently being carried out across the country.

Anti-police terror police swooped on a number of suspects in the hours after the attacks as the Belgian Foreign Ministry said many of those behind the terror plot are 'still at large'.

Bravery: People injured are treated, comforted and given water by the emergency services as they help the wounded

Witnesses described horrific scenes inside the terminal in the wake of the suspected suicide blast

Two blasts detonated near check-in desks at Brussels Airport at 8am (7am GMT) in a suspected suicide bombing

The ISIS (file photo) statement described how its 'soldiers' allegedly carried out the attacks this morning

At least two people in Brussels were arrested outside the city's North railway station, a mile from the Maelbeek subway.

A third suspect has been arrested on a train near Amsterdam and a suspect package at Gard du Nord in Paris delayed Eurostar services this afternoon.

Another man was also taken into custody at by armed police at Brussels South railway station near the suburb of Schaerbeek.

MailOnline revealed earlier today that security services already had CCTV of one of the Brussels airport bombers including the moment he detonated his suicide belt.

Surrounded: Two men on their knees with hands on their head are held in Brussels as the authorities

Drama: Two men were pinned to the ground by armed police and special forces as the hunt for members of the terror cell behind today's bombings in Brussels started

Interventions: The arrests came as the authorities start to round up any people deemed a risk to the public, including here at Brussels North station - a mile from the Maelbeek bombing

Across the border: A suspect with his hands up is arrested as he is taken off a train because of suspicious activity at Hoofddorp Station in Amsterdam

TERROR TIME LINE: THREE BOMB BLASTS IN BRUSSELS IN JUST 79 MINUTES 8am: Shots heard at Zaventem international airport before someone shouted in Arabic and two explosions rocked the departure hall 8am onwards: Terrified passengers seen streaming out of the terminal while flights are diverted 8.30am: Witnesses describe blasts so powerful victims were thrown in to the air, Airport is closed 9.19am: A third bomb blast rips through Maalbeek Metro station killing 20 more people 9.30am: Belgium moves to its highest level of terror alert 10am: The European Commission tells staff to stay home or in their offices 10.30am: The crisis centre asks residents to stay put, and all public transport systems close down 10.30am: Security reinforced in airports, train stations and public transport systems in Paris while airports in Frankfurt, London, Moscow and the Netherlands 11am: Two suspects arrested one mile away from Metro Station blast 11.15am: Police and soldiers reinforce security around Belgium's nuclear power plants. 12pm: A Kalashnikov and unexploded suicide bomb vest are found in the rubble at the airport 12pm: Belgian prosecutors confirm that the three explosions were terror attacks 1.50pm: Dutch Police stop international train from Brussels to Amsterdam at a station just one stop from Holland's Schiphol Airport as a precaution and search the train and its passengers 3pm: Belgian TV station reports at least one of the bombs at the Brussels airport contained nails 4.40pm: ISIS ISIS group claimed responsibility for the attacks in Brussels Advertisement

Passengers and airport staff are evacuated from the terminal building after explosions at Brussels Airport in Zaventem near Brussels

The ISIS statement described how its 'soldiers' allegedly carried out the attacks this morning.

It addded: 'By the grace of Allah and his goodness, a secret cell of soldiers of the caliphate, which Allah gave power and victory, has launched an attack in the direction of Belgium which has not ceased to combat Islam and Muslims.

'Allah has thus given victory to the hands of our brothers and thrown fear and terror in the hearts of our enemies, in the middle of their land.

'In effect, a number of soldiers of the caliphate wearing explosive belts bombs and military guns and choosing their targets with precision in the Belgian capital Brussels, have launched attacks inside the Zaventem airport and a metro station to kill a largue number of our enemies.

'They then detonated their suicide belts in the middle of the crowds. the death toll has risen to 40 dead and at least 210 injured among the citizens of the crusader states.'

In London: Travellers stand in a long queue at St Pancras, after all trains in and out of Brussels were suspended this morning

Prime Minister David Cameron has called a meeting of the government's emergency COBRA committee, and said Britain would 'do everything we can to help'.

Britain's official terrorist threat level stands at 'severe', the second-highest level on a five-point scale, meaning an attack is highly likely.

Mr Cameron said: 'These are appalling and savage terrorist attacks and I've just spoken to the prime minister of Belgium to give our sympathies and our condolences to the Belgian people.

'We absolutely stand with them at this very difficult time. These were attacks in Belgium; they could just as well be attacks in Britain or in France or Germany or elsewhere in Europe.

'We need to stand together against these appalling terrorists and make sure they can never win. I've also made sure that we've offered every support to the Belgian security and policing and intelligence forces at this time.'

David Cameron said the world must stand firm in the face of terrorism, saying on Twitter: 'We will never let them win'

All flights between the UK and the main airport in Brussels have been cancelled for the rest of today, and Brussels Airlines cancelled 25 flights between UK airports - Heathrow, Manchester, Birmingham, Bristol and Edinburgh - and Brussels.

Eight flights between Heathrow and Brussels were cancelled by British Airways.

In London, police were carrying out extra patrols of airports, and advising passengers to check with their airlines before travelling.

A Gatwick spokesman said: 'The safety of passengers and staff at Gatwick is the airport's absolute priority. As a result of the terrible incidents in Brussels, we have increased our security presence and patrols around the airport.'

Similar steps were being taken at other airports, including Heathrow, Glasgow, Edinburgh and Manchester, and Heathrow was advising passengers to check their flight status with their airline.