Volcanic ash from Iceland has grounded flights across Europe and could affect plans by world leaders to attend the funeral of Polish President Lech Kaczynski in on Sunday. (Jon Gustaffson/Associated Press)

A spreading cloud of volcanic ash emanating from Iceland is threatening to derail the funeral of Polish president Lech Kaczynski and his wife, Maria, on Sunday in Krakow.

The volcanic ash has disrupted European air travel and forced Poland to close most of its airspace on Friday.

As a result, it is unclear whether state leaders planning to attend the funerals will make it.

Among those who said they would come are Prime Minister Stephen Harper, U.S. President Barack Obama, Russian President Dmitry Medvedv and German Chancellor Angela Merkel.

"We continue to monitor the situation closely," Harper's spokesman, Dimitri Soudas, said Friday. Officials at the White House are also "keeping an eye on" the situation, White House deputy press secretary Bill Burton said Thursday.

Jacek Sasisn, a spokesman for the Presidential Palace, told a popular Polish commercial radio station that no decision has yet been made about delaying Sunday's funeral, and that any delay would be an "absolute last resort."

Kaczynski's family is urging that the state funeral be held as planned.

"It is the will of the family that, under no circumstance, the date of the funeral be changed," Sasisn said on the family's behalf.

Black boxes investigated

Meanwhile, Polish investigators have begun examining one of three black boxes from the wreckage of the Tu-154 plane that crashed Saturday while approaching the airport in Smolensk, Russia, killing the president and 95 others.

Workers prepare a stage for a funeral service for the victims of the presidential plane crash in Warsaw on Thursday. (Markus Schreiber/Associated Press)

Russian and Polish investigators have already said human error was the likely the cause of the crash, but the black box will be examined for more or other possible clues.

The other black boxes are being examined in Russia.

The investigation into the crash is moving fairly quickly, aviation experts said, but some Poles have complained about a lack of public information, including the transcript of conversation in the cockpit before the accident.

Jerzy Artymiak, spokesman for Polish military prosecutors, said the other two boxes are still being examined in Russia.

Investigators had hoped to disclose contents of the flight recordings on Thursday, but Artymiak said they now plan to wait until after the weekend memorial ceremonies.

Poland is preparing a tradition-laden funeral for Kaczynski and his wife. They were headed to the Smolensk airport for a commemoration of the systematic execution of thousands of Second World War Polish army officers by the forerunner of the Soviet secret police in 1940 when the crash happened.

With files from CBC News