Angelopoulos-Daskalaki (left) is under pressure to deliver

Organisers have expressed concern over whether all the venues in the Greek capital will be ready by 13 August.

Rogge said: "There's still a lot to do - we have only five-and-a-half months.

"Our experts say if the pace of the work continues, there is enough time. This is a strong message that's been conveyed to the organisers."

Rogge's pointed comments contrasted sharply with more upbeat assessments he had given in recent weeks.

Made at a joint meeting of the IOC executive board and the Association of National Olympic Committees in Athens, they were clearly designed to spur on the organising committee.

It will take a great effort to get these things done Athens 2004 president Gianna Angelopoulos-Daskalaki

A number of key construction projects are yet to be completed, including a giant steel-and-glass roof over the main Olympic stadium.

Athens 2004 president Gianna Angelopoulos-Daskalaki said her team had made considerable progress with venues, transport infrastructure and the staging of test events.

But she admitted tram and rail lines, the stadium roof, the swimming venue and improvements to the marathon course still needed addressing.

"It will take a great effort to get these things done," said Angelopoulos-Daskalaki.

"But we haven't come this far to let challenges like these stop us now."

The Athens Games will be Rogge's first since his election as president in 2001.

The Olympic stadium roof may not be ready on time

He also issued a warning to Athens about preparations as head of the IOC's oversight panel in 2000, and said that had brought about improvements.

But it is rare for an IOC president to raise doubts about the success of an Olympics so close to the opening date.

The IOC is sending experts to Athens on almost a weekly basis and meeting with the organisers several times a month.

"If the Athens Games run smoothly, and, I repeat, if the Games run smoothly, then we will see the Olympic Solidarity budget rise," said Rogge.

Olympic Solidarity manages the share of Olympic television and marketing rights allocated to the national committees.

Greece has budgeted more than �430m for security, more than three times what was spent to make the 2000 Sydney Games safe.

Anti-Olympic activists firebombed two government vehicles on Thursday to coincide with the IOC meetings in Athens.

But Rogge said the Greek government was taking "all possible scenarios" into account in security planning.