BP has reported progress in its struggle to plug its gushing Gulf of Mexico oil well, but President Barack Obama cautioned there was no "silver bullet" solution to the biggest oil spill in US history.

Trying to show leadership in the face of growing criticism over his handling of the spill, President Obama toured the Louisiana Gulf coast, where oil has seeped into wetlands and closed down the lucrative fishing trade, threatening thousands of jobs.

"You will not be abandoned. You will not be left behind. We are on your side and we will see this through," Obama said in a televised statement after meeting local and state officials and inspecting the oil spill damage to the coastline.

Frustrated Gulf Coast residents have loudly criticized federal authorities for being slow to act and offering too little assistance. The White House vigorously disputes this, insisting it has mounted the largest response in history.

BP Chief Executive Tony Hayward said the so-called "top kill" procedure, in which mud is pumped into the seabed well shaft, was showing some signs of success in choking off the five-week-old leak that has already spewed millions of gallons of oil into the Gulf.

But the success of the operation, never attempted at such depths, was still uncertain and it would be another 48 hours before the company knew if it had worked, he said.