Reuters reports this follows a spate of child deaths linked to similar syrups in some countries last year, but the WHO statement did not say whether any children in the Marshall Islands or Micronesia had fallen sick.
The WHO said samples from a batch of imported cough syrup was contaminated with unacceptable amounts of diethylene glycol and ethylene glycol.
These are toxic to humans when consumed and can prove fatal.
Guaifenesin is an expectorant used to relieve chest congestion and the symptoms of cough.
RNZ Pacific's Marshall Islands correspondent said the Ministry of Health put out alerts earlier this month and removed stock of the cough syrup from the pharmacies of its two hospitals.
Samples from the suspect batch were analysed after being reported to the WHO on April 6.
Manufacturer, QP Pharmachem's Managing Director said the company took the Indian government's permission to export 18,000 syrup bottles only to Cambodia.
The syrup was also distributed in India, he said, adding that no complaints had been received so far.
Marketer Trillium Pharma did not immediately respond to a request for comment.
In January, the WHO called for immediate action to protect children from contaminated medicines after a spate of child deaths linked to cough syrups last year.
In 2022, more than 300 children - mainly aged under five - in Gambia, Indonesia and Uzbekistan died of acute kidney injury, in deaths that were associated with contaminated medicines.