NEW YORK (CNN/Money) - Several major retailers have expanded the number of states in which they collect sales taxes from their online customers, according to a published report Monday.

About 10 retailers with online stores, including Wal-Mart, Target, and Toys "R" Us, increased the number of states where they will charge a sales tax, a move that is likely to pressure additional states and pure Internet retailers, such as Amazon.com, to start collecting sales tax, the Wall Street Journal reported.

Under the agreement, which includes 37 states and the District of Columbia, the states will not pressure the retailers for back taxes, the paper reported.

Internet retailers have not typically collected a sales tax unless they have a physical presence, or nexus, in a particular state. But retailers are blending their physical and Web operations more frequently and this increases the likelihood that they will have to collect sales taxes, the Journal reported.

A spokesman for Seattle-based Amazon.com said complying with the agreement is difficult because taxes vary in particular states, such as Colorado where it fluctuates from 4.3 percent to 8 percent, and the company does not know what to charge.

For most of its customers, Amazon.com only charges sales taxes in Washington state and North Dakota, where it has its headquarters and a customer-service center, according to the paper.