Hi, and welcome to the forum.

First of all, if you're coming from the US, and you're talking about one of those tax deferred retirement funds (IRA, specifically), the restrictions may come more from the IRS than from the Singapore government.

It will also make some difference whether you're working on the local payroll or on the expat payroll of your employer back in the US. On an expat payroll, your employer generally is allowed to maintain your payments into any benefit plans (including retirement plans) while you are on assignment overseas. But if you are making use of the overseas earned income exclusion, you won't be able to make contributions to an IRA while your are overseas. (Not sure about a 401K plan - check with your employer if that's what you have.)
Cheers,
Bev