Can EP holder assigned overseas?

You will pay tax in Singapore as per the fixed earnings mentioned in your offer letter. Any tax needs to pay in that onsite country, can be verified under their tax provision.

What Surya wrote above is not true (unless the rules have changed, as my experience is many years old):
I was seconded to Indonesia by my Singapore employer for 6 months and paid income tax there (although I received the pay in Singapore). I declared this truthfully to IRAS and was told income for work performed overseas is not taxable in Singapore - so I paid no tax on it here.
A few years later, I worked for a Singapore employer in Taiwan. Again I received the pay in my Singapore bank account and they even paid CPF on it (I had become PR in the meantime). IRAS again told me that I do not need to declare this income, as it is not taxable here.

Agreed Beppi, in your case you have been working for a long assignment in a foreign country and needs to provide proof that during that period you are not in Singapore office.

Otherwise, there is a set of rules in place, read below (copied from iras site):

Overseas income is taxable in Singapore when:

Your overseas employment is incidental to your Singapore employment. That is, as part of your work here, you have to travel overseas.
Example: Overseas Services Incidental to Singapore Employment
You work in Singapore for a  foreign employer  .
Your overseas employment income is for services rendered in Singapore.
You have a trade / business in Singapore and you are carrying on a trade / business overseas which is incidental to your Singapore trade.
You have received the income in Singapore through a  partnership  in Singapore, unless the income qualifies for exemption*.
You received service income from overseas, unless the income qualifies for exemption*.

*Please refer to the guide on Tax Exemption for Foreign-Sourced Income (614 KB) for details.
You are employed outside of Singapore on behalf of the Government of Singapore.