BP said earlier on Friday that the containment cap could be able to capture upward of 90 per cent of the oil but that it would take 48 hours to know how well the system was working.
The operation follows a series of unsuccessful attempts to cap or contain the gushing leak since an explosion on April 20 tore through the BP-leased Deepwater Horizon rig just off the Louisiana coast.
John Terret, Al Jazeera's correspondent in Venice, Louisiana, said: "[BP] said that the cap is in place and its four vents will be tightened slowly until the flow is reduced to next to nothing.
"[However], even when that cap its secure there will still be oil leaking into the Gulf.
"So that it why BP is saying that it is not going to be until August that the leak is contained."
Barack Obama, the US president, has cancelled a trip to Australia and Indonesia in order to visit Louisiana on Friday, his third trip there since the spill began.
Obama said in an interview on Thursday that while the US Coast Guard was responsible for co-ordinating relief efforts with BP, the government was reliant on the London-based energy firm for its expertise and equipment to plug the broken well.
Terret said: "Obama is under enormous pressure. The people of Louisiana blame his administration for letting them down.
"He was only on the ground here during his last visit for two and a half hours and people said that that was not enough."
BP has already spent well over $1bn on its response to the spill and has attempted to reassure investors that the firm is not at threat of insolvency due to the spill.
Tony Hayward, BP's chief executive, said that it had plenty of money to meet its obligations, including $5bn in cash and additional credit if necessary.
Hayward said that it had "considerable firepower" to cope with the costs, although he acknowledged that those arrears would be "severe" and long-running.
BP was formally reassuring shareholders for the first time since the spill started, addressing the need to now improve the company's safety reputation and clean up the Gulf coast.
Carl-Henric Svanberg, the BP chairman, said: "We will meet our obligations both as a responsible company and also as a necessary step to rebuilding trust in BP as a long term member of the business communities in the US and around the world."
"This is in the interest of all our stakeholders."