Allegations of corruption continue to dog British pharmaceutical company GlaxoSmithKline (GSK).

The latest reports come from Syria, where the drugmaker and its distributor have been accused of paying bribes to secure business, according to a whistleblower's email reviewed by the news agency Reuters.

Claims against GSK, biggest pharmaceutical player in the UK, in this case date back to 2010. The allegations relate to the company's former consumer healthcare operations in Syria, which closed in 2012 due to the worsening civil war in the country.

"We have zero tolerance for any kind of unethical behavior. We will thoroughly investigate all the claims made in this email," GSK said in a statement.

The company has been rocked by corruption allegations since July 2013, when Chinese authorities accused it of funneling up to 3 billion yuan ($480 million) to doctors and officials to encourage them to use its medicines. The former headof the drugmaker's China business was accused in May of this year of being behind those bribes.

Since then, smaller-scale bribery claims have surfaced in other countries and GSK is now investigating possible staff misconduct in Poland, Iraq, Jordan and Lebanon.

In Syria, allegations center on the company's consumer business, including its popular painkiller Panadol and oral care products.

Although rules governing the promotion of non-prescription products are not as strict as for prescription medicines, the email from a person familiar with GSK's Syrian operations told Reuters the alleged bribes in the form of cash, speakers' fees, trips and free samples were in breach of corruption laws.

In addition, the email said GSK had engaged in apparent Syrian export control violations, including an alleged smuggling scheme to ship the drug component pseudoephedrine to Iran from Syria via Iraq. Pseudoephedrine is regulated as a precursor for making methamphetamine.

GSK said it would investigate this matter along with the bribery claims.

Britain's Serious Fraud Office launched a formal criminal investigation into GSK's overseas activities in May and the US Department of Justice (DOJ) is investigating it for possible breaches of the Foreign Corrupt Practices Act (FCPA).

GSK has overhauled its marketing policies in the wake of concerns about possible past misconduct. It aims to become the first company in the industry to stop paying outside doctors to promote its products, Reuters reports.