RIO DE JANEIRO - Construction work is "advanced" at three Rio 2016 venues as the city steps up its Olympic preparations, local organizers said Thursday.

Structures for spectator stands have already been built at indoor arenas earmarked for basketball, judo, fencing, taekwondo, wheelchair basketball, wheelchair rugby, boccia and judo, according to the Rio 2016 website.

Work on the adjacent Olympic tennis center, which will seat 10,000 people, is also "taking shape", organizers said.

"Significant progress has been made and we are happy to see that work is proceeding as planned, and in some cases, is ahead of schedule," said Rio 2016 Infrastructure Integration Director Alexandre Techima.

"Venue construction has progressed a lot over the past few months and the structures of a number of arenas are already visible, such as the Carioca Arenas and the Olympic Tennis Centre, as well as the International Broadcasting Centre (IBC), Main Press Centre (MPC) and hotel."

Meanwhile, foundations are being laid for the Olympic Velodrome and Aquatics Stadium.

Rio Olympic Arena and Maria Lenk Aquatics Centre are scheduled to undergo refurbishment work in the first quarter of 2015.

Organizers said Barra Olympic Park already boasts more than 10.5km of drainage, 5.3km of sewage pipes, 8.3km of water supply lines, 5km of fire prevention lines, 5km of lighting, 9.9km of medium-voltage power lines and 21.9km of telecommunications lines.