Jeremiah is imprisoned (and rescued) again (Jeremiah 38)

Jeremiah's prophesies turn to saying that those that surrender to the Babylonians will be spared while those who remain in Jerusalem will die.  He is arrested for inciting desertion and put into a cistern.  Before doing so, his arresters ask for permission from the king to do this, who says he is powerless to stop it.

Later, Jeremiah loyalists then plead with the king for Jeremiah's release.  This time, the king assigns 30 soldiers to help lift Jeremiah out of the cistern.  The king then has another private conversation with the king and Jeremiah advises surrendering Jerusalem otherwise everyone will be killed.

The king then advises Jeremiah to lie in case his enemies ask what he told the king.  Sure enough, Jeremiah's enemies inquire about his conversation with the king and Jeremiah tells the lie that the king instructed him to do.

I have frequently wondered about the morality of lying in times like that.  Is there a warfare or necessity exception to lying?  What about secret agents behind enemy lines, they must lie all the time.

The issue was similarly raised when Joshua's spies scoped out Jericho and Rahab lied to protect them.  Here, Jeremiah, a major prophet of God, lies to his enemies under specific orders from the king.