US officials are facing mounting pressure to explain the downing of unidentified objects flying over North America in recent days. The US military says it is unsure what the objects were - and how they were able to stay aloft. President Joe Biden ordered another object to be downed on Sunday. It’s the fourth such incident since the downing of a suspected Chinese spy balloon on 4 February. A US military commander said the object could be a "gaseous type of balloon" or "some type of a propulsion system". But he added he could not rule out that the objects were extra-terrestrials. Meanwhile Beijing says the United States has been flying balloons over Chinese airspace. The foreign ministry said this happened on more than ten occasions since last January.