SZCZEKOCINY, Poland - Poland's government has insisted that rail travel is safe in the country despite a train collision that killed 16 people. The assurances come months before masses of sports fans will enter the country for a major soccer tournament -- many of whom will crisscross the nation by train.

Saturday night's crash, Poland's mostly deadly rail tragedy in more than two decades, raised new questions about the safety of a state-run rail network, which has undergone modernization in recent years. Poland still has a rail system marked by the legacy of the communist decades, but has been working to upgrade trains and tracks.

The trains collided head-on in a shower of sparks and mangled metal, killing 16 people and injuring dozens more near the southern town of Szczekociny, just north of Krakow. Both trains inexplicably ended up running on the same track. Polish leaders said it was the worst rail tragedy since 16 people were killed in a 1990 collision near Warsaw.

Some routes today are notorious for being slower than they were even before World War II -- and the economically dynamic young member of the European Union has been pushing to change this even as it builds skyscrapers, highways and stadiums. Several of the construction projects have been accelerated by the coming Euro 2012 soccer championship, which starts in June.

Transport Minister Slawomir Nowak insisted Sunday train travel is safe and that the government makes safety a priority as it improves the system. The collision occurred on a stretch of track that was recently modernized, but officials said it was too early to speak about a cause.

"I really believe that the train system -- not only in Poland but all of Europe -- is still very safe," Nowak said. He said those who plan to use the trains this summer during Euro 2012 should not worry. Poland is co-hosting the three-week tournament with Ukraine, and games will be held in several Polish and Ukrainian cities, which will force some fans to travel large distances -- either by train, plane, bus or car.

President Bronislaw Komorowski called for two days of national mourning Monday and Tuesday, meaning that flags will fly half-staff at public buildings, and concerts and sporting events will be cancelled. Poland's Roman Catholic bishops also called for prayers for those killed and the injured -- an appeal that will certainly be heeded in the predominantly Roman Catholic country.

Interior Minister Jacek Cichocki said rescue officials believe they have retrieved all bodies from the wreckage because sniffer dogs have not found any other traces of corpses.

An American woman was killed, while other foreigners -- Ukrainians, Moldovans and a Czech citizen -- were among those brought to hospitals.

The U.S. was among several countries to send condolences, with Washington offering assistance.

"The president sends his deepest condolences to the Polish people and to the families whose loved ones were killed or injured in yesterday's train accident in Poland," a spokesman for the White House's National Security Council, Tommy Vietor, said. "As a close ally of Poland, the United States stands with the Polish people and is prepared to provide whatever assistance may be helpful."

Sixty people were initially hospitalized after the crash, and 51 remained hospitalized Sunday afternoon, Health Minister Bartosz Arlukowicz said. Three of them were in serious condition, while the others had lesser injuries, he said.

Dr. Slawomir Milka, on duty at a hospital in Zawiercie, said those in serious condition were stable.

The crash turned cars at the front of each train into heaps of mangled metal and toppled others on their sides. A woman living in a house nearby said she was standing at her window when the two trains collided, creating a "terrible, terrible noise -- like a bomb going off."

"So I ran out of the house, and on one side I saw train lights and one the other side I saw train lights, and in the middle sparks," Anna Sap said. "People from the train starting crying, 'Help, help!' So we and the neighbours ran to them. Some of them smashed windows to let them out."

Her husband Grzegorz Sap added that people began emerging from the train "with hand luggage and in shock. They had no idea where they were."

An unnamed passenger interviewed on the all-news station TVN24 said he felt the force of the collision.

"I hit the person in front of me. The lights went out. Everything flew," he said. "We flew over the compartment like bags. We could hear screams. We prayed."

Rafal Krupa, a council member in the nearby town of Zawiercie, said emergency workers got to the site as quickly as possible, but that in the first moments it was difficult for them to determine where the crash had occurred. Passengers on the train "began calling in to the emergency services, but they weren't really aware of where they were," he said.

One train was travelling from the eastern city of Przemysl to Warsaw in the north, while the other -- an Intercity train travelling 95 kph (60 mph) on the wrong track -- was heading south from Warsaw to Krakow, officials said. The speed of the first one, a regional train, was not yet known, Nowak said.

The country's most deadly train disaster post-World War II dates back to 1980, when 65 people were killed when a freight train collided with a passenger train near Otloczyn.