Police have closed a Queanbeyan street outside an apartment block, conducting an "operation" that appeared to be a siege.

NSW Police have refused to confirm the nature of the operation but police and firefighters have entered and exited the building over several hours while paramedics wait outside.

Police cordoned off Donald Road between Margaret Street and Anne Street at about 7pm, but first arrived at 2pm.

Neighbours described the presence of emergency services growing through the afternoon before police closed the street.

NSW Police would only say that it was conducting an operation in the apartment block.

Police remained inside the building as of 8.50pm.