Whenever the Secretary of the Army, in pursuance of authority conferred on him by law, causes proceedings to be instituted in the name of the United States for the acquirement by condemnation of any lands, easements, or rights of way needed for a work of river and harbor improvements duly authorized by Congress, the United States, upon the filing of the petition in any such proceedings, shall have the right to take immediate possession of said lands, easements, or rights-of-way, to the extent of the interest to be acquired, and proceed with such public works thereon as have been authorized by Congress: , That certain and adequate provision shall have been made for the payment of just compensation to the party or parties entitled thereto, either by previous appropriation by the United States or by the deposit of moneys or other form of security in such amount and form as shall be approved by the court in which such proceedings shall be instituted. The respondent or respondents may move at any time in the court to increase or change the amounts or securities, and the court shall make such order as shall be just in the premises and as shall adequately protect the respondents. In every case the proceedings in condemnation shall be diligently prosecuted on the part of the United States in order that such compensation may be promptly ascertained and paid.Provided