The UK anti-doping agency says it sent two investigators to Kenya to look into claims four British athletes were using blood-booster EPO. Meanwhile one more Russian athlete has been cleared for Rio but 135 must wait.
UKAD was responding to allegations made by German broadcaster ARD and British newspaper The Sunday Times, who say three Kenyan men told them in an undercover sting that they had either provided or administered EPO to the British athletes. No athletes were named.
The organization's chief executive Nicole Sapstead says the investigators "are currently in Kenya pursuing a number of lines of enquiry.
"UK Anti-Doping has reviewed the evidence presented to us by The Sunday Times and it is of grave concern and of significant interest," she added.
The ARD documentary appeared to show boxes of EPO and syringes in a bin at a Kenyan High Performance Training Centre in Iten which top global athletes, including some from Britain, use for altitude training.
Russian athletes must wait
Meanwhile the IAAF on Sunday cleared long jumper Darya Klishina to compete as a neutral athlete in international competition, meaning she is eligible to compete in the Olympics in Rio next month.
However, Klishina - a US-based long jumper - was the only exception granted, meaning 135 more, including pole vault star Yelena Isinbayeva, must wait for an impending court decision from the Court of Arbritration for Sport.
"The IAAF Doping Review Board has today agreed that Russian long jump athlete Darya Klishina meets the exceptional eligibility criteria to compete in international competition as a neutral athlete," the IAAF said in a statement.
"The Doping Review Board received a total of 136 applications from Russian athletes seeking exceptional eligibility to compete in an individual capacity," the statement said.
The decision on Klishina follows a similar one in the case of whistleblower Yuliya Stepanova, who was granted the right to compete as a neutral last month. She took part in the European Championshps in Amsterdam but was injured in her heat.
As is the case with Stepanova, Klishina's participation in Rio is subject to acceptance by the International Olympic Committee.