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Flights in and out of airports in Tel Aviv, Dubai, Doha and other international hubs in the region have been suspended.
The UK Foreign Office is warning British citizens against all but essential travel to Bahrain, Kuwait, Qatar and the United Arab Emirates (UAE). Those already there have been advised to shelter.
Iran has launched attacks on the Gulf states as part its retaliation to US and Israeli strikes which began early on Saturday. Long-haul travel is being affected more widely, with Heathrow urging travellers to check with their airlines.
British government officials are understood to be formulating plans to potentially evacuate UK nationals from the Middle East.
But the timings of such a move remain unclear, as much of the airspace in the region remains closed.
Emirates has suspended its operations in and out of Dubai until 15:00 local time (13:00 GMT) on Monday due to airspace closures, while Etihad has suspended flights out of Abu Dhabi until 02:00 local time.
One person has been killed and 11 others injured at airports in Dubai and Abu Dhabi since the strikes began. Four of those injuries were among members of staff at Dubai International, the world's busiest airport by passenger traffic.
British Airways has cancelled services to Tel Aviv and Bahrain until Wednesday.It said services between Heathrow and Abu Dhabi, Amman, Bahrain, Doha, Dubai or Tel Aviv could be affected for several days.
Richard and Hannah from London had been en route to Oman but are now stuck in Bahrain.
"In the early hours of this morning a drone attacked the airport so we can't reach Oman this evening as planned," Hannah said on Sunday, describing an "uneasy 24 hours".
Given the situation in the region could escalate, they are now looking to return home, Richard said.Virgin Atlantic suspended services between London and Riyadh and Dubai over the weekend.
It has warned that flights to India, Saudi Arabia and the Maldives may take longer due to them being rerouted around the affected region.It is among a number of airlines across the world that have had to cancel or reroute flights to avoid the closed or restricted airspace.
Airspace over Iran, Israel, Iraq, Qatar, Bahrain, Kuwait, and Syria remained closed on Sunday, with partial closures in the UAE and Saudi Arabia.Jordanian and Lebanese airspace remains open but there is limited flight activity.
Tracking shows flights between Europe and Asia travelling via Saudi Arabia or the Caucasus.
Emma Belcher and her husband Vic were on their way back to Heathrow from a holiday in the Maldives via Dubai when their connecting flight was cancelled."There is absolutely no information about when they might open airspace so we don't know how long we'll be here," she said.
"We were really looking forward to getting home to see the children as we haven't been away without them before."
Steve Rudderham and his wife had been on their way to the Maldives to celebrate their wedding anniversary but were stuck in Doha where they had been due to have a stopover.
"As the days go on and the vacation is eroded, we're looking at plans to just to get back home," he said.Flights that would have gone over the affected region have been diverted, causing delays
Qatar Airways said its operations remained temporarily suspended due to the closure of Qatari airspace, with a further update to be provided at 09:00 local time on Monday (06:00 GMT).Qatar's defence ministry said it had intercepted Iranian missiles after explosions were heard in Doha.
The UAE also said it had intercepted Iranian missiles and drones, while footage appears to show the moment a US facility in Bahrain was hit.
Kuwait said it had been targeted by Iranian missiles and drones. Explosions were heard at Erbil airport in Iraq early on Sunday morning.The UK Foreign Office is warning "against all travel to Israel and Palestine", and against all but essential travel to Bahrain, Qatar, Kuwait and the UAE.
British people in those areas have been urged to register their presence with the Foreign Office, with more than 76,000 people having already done so, mostly in the UAE.
For British nationals in Oman, the Foreign Office advises those in Duqm to shelter in place, and those in Salalah to leave as soon as possible if commercial means allow and follow advice from local authorities.
UK nationals in Saudi Arabia have been told to "remain indoors in a secure location", while those in Jordan, Syria, Lebanon, Iraq, Yemen, and Turkey have been instructed to "remain vigilant" and take shelter if advised to do so.
The Foreign Office has urged people in Pakistan to be careful around protests, demonstrations, rallies and religious gatherings, after deadly pro-Iran protests broke out in several cities. It also instructed staff in the country to restrict their movements.
US President Donald Trump cited a failure to reach an agreement to limit Iran's nuclear programme and regime change as among the reasons for launching the attack.
Prime Minister Sir Keir Starmer spoke to Trump on Saturday, having earlier said that British planes were "in the sky" over the Middle East as part of a defensive operation "to protect our people, our interests and our allies".
Along with the leaders of France and Germany, he urged Iran to refrain from "indiscriminate" strikes.