The funeral of Polish president Lech Kaczynski - due to take place on Sunday - may be delayed after Poland's airspace was closed because of volcanic ash.
"I think its a very serious alternative which must be taken into consideration," presidential aide Jacek Sasin told Poland's RMF commercial radio Friday.
Kaczysnki was among 96 people killed in a plane crash last Saturday.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Dozens of foreign dignitaries including US President Barack Obama are due to attend his funeral in the southern city of Krakow.
Poland closed most of its air space on Friday, with the exception of Krakow and Rzeszow airports, due to a cloud of ash spreading from an Icelandic volcano.
The shutdown took effect at 8am (1600 AEST) , said Grzegorz Hlebowicz, a spokesman for the Polish civil aviation authority, cited by the PAP news agency.
Ash cloud 'could take days to clear'
"That means that aircraft cannot take off or land. Air traffic is only possible from Krakow-Balice airport and from Rzeszow airport," both in the south of Poland, he said.
Experts have warned the fallout from the Eyjafjallajokull volcano in southeast Iceland, which covered the skies of northern Europe on Thursday, could take several days to clear.
Polish officials said nearly 80 planes carrying world leaders were due to land at Krakow over a six-and-a-half hour window on Sunday morning ahead of the funeral of the Polish leader and his wife, Maria.
More dignitaries were also set to land at Pyrzowice, an airport near the city of Katowice, about 100km (60 miles) from Krakow.
Russian President Dmitry Medvedev, French President Nicolas Sarkozy, German Chancellor Angela Merkel and EU president Herman Van Rompuy are among a host of other European and world leaders set to attend.
Britain's Prince Charles and Vatican envoy Cardinal Anglo Sodano are also on the list of high-level participants.
The funeral is due to be held in the cathedral crypt of Krakow's Wawel castle, where Poland's past kings and national heroes are laid to rest.
