Some Singapore Press Holdings (SPH) websites such as straitstimes.com and AsiaOne were not fully accessible on Monday.
Also, access to the ST website late on Sunday night was intermittent.
A spokesman for SPH, which publishes all SPH websites, acknowledged that some readers had had difficulties accessing the websites on the two days and apologised for the inconvenience.
SPH added that its information technology division is "investigating the matter".
The sites are fully accessible now.