JAKARTA (Reuters) - Indonesians caught launching big, unmanned hot air balloons could face up to two years in jail or a fine of up to $35,000, authorities said, after balloons launched to celebrate the end of the Muslim fasting month forced pilots to divert flights.

Officers show an unmanned hot air balloon, used in traditional festivities for celebrating Eid al-Fitr, during a press conference at AirNav Indonesia, in Yogyakarta , Indonesia June 16, 2018 in this photo taken by Antara Foto. Antara Foto/ Andreas Fitri Atmoko/via REUTERS

Balloons, along with fireworks have long been part of festivities like Eid in Indonesia, home to the world’s largest Muslim population, but with modern materials and designs they have become much bigger.

Some are up to 10 meters in diameter and more than 20 meters high, the transportation ministry said in a statement on Monday that warned of the penalties.

Air Transportation Director General Agus Santoso said some balloons had been “reaching cruise altitude level at 10,000 meters above sea level.”

The transportation ministry issued its statement after pilots sighted dozens of balloons in the skies over the archipelago in the days after Eid, which fell on Friday in Indonesia this year. Most of the sightings were over the provinces of Central and East Java.

Novie Riyanto, head of AirNav Indonesia, said the balloons posed a “very significant” hazard, and there had been 84 sightings over the weekend, prompting complaints from dozens of domestic and international pilots.

“Yesterday a great deal of traffic could not travel on routes, and had to ascend to avoid these balloons,” he said in the statement.

Riyanto added that the airspace over Java was the fifth-most crowded in the world and served as an international crossing.

As a result of the balloons, AirNav Indonesia had blocked off part of that air corridor for other aircraft, he said.

“This is a major disruption.”

Indonesia has a patchy air safety record, with 12 accidents in 2016 in which 30 people died, according to the Aviation Safety Network website.

Santoso urged the public to follow rules that balloons must be tethered and flown at a maximum height of 150 meters or lower near airports.

Police in East Java had confiscated 43 balloons before they could be launched, spokesman Frans Barung Mangera said in a statement sent by text message on Monday.